---
title: "Glossary"
description: "Definitions of key Wavelength, Ark, and Lightning Network terms used throughout the documentation."
canonical: https://wavelength.lightning.engineering/glossary/
---

> Docs index: https://wavelength.lightning.engineering/llms.txt

# Glossary

### Ark

Ark is the shared off-chain protocol layer Wavelength builds on. Users hold VTXOs (virtual UTXOs) that settle on-chain in batched rounds, so payments are instant and cheap while remaining self-custodial, and users can always exit to on-chain Bitcoin unilaterally. This underpins boarding, rounds, VTXOs, and leaving Ark, which have their own entries.

### Batch swap

A batch swap refreshes VTXOs that are nearing expiry by forfeiting them in the current round in exchange for new VTXOs in a fresh batch. The new outputs reset the batch lifetime clock, so you can keep funds off-chain without returning to the Bitcoin base layer. Wavelength may perform batch swaps automatically as part of routine wallet maintenance. See [Balances & VTXOs](/concepts/balances-and-vtxos/) for how VTXO expiry fits into the lifecycle.

### Boarding

Boarding is how you enter Ark from the Bitcoin base layer. You spend an on-chain UTXO as an input to the operator’s next batch transaction and receive one or more VTXOs in return. Once boarding completes, your funds live off-chain and can be sent, swapped, or withdrawn through Ark operations. See [Balances & VTXOs](/concepts/balances-and-vtxos/) for the full boarding-to-VTXO flow.

### Boarding address

A boarding address is an on-chain Bitcoin address your wallet derives for deposits that will board into Ark. Send base-layer bitcoin to this address; Wavelength tracks the deposit and boards it at the next round tick (typically on the order of \~90 seconds). Use `wallet.deposit()` as described in [Get a deposit address](/guides/get-a-deposit-address/).

### BOLT 11

BOLT 11 is the standard Lightning invoice format. Every Lightning payment Wavelength sends or receives is a BOLT 11 invoice, so the wallet interoperates with any Lightning wallet, exchange, or app. See the [Lightning receive](#lightning-receive) and [Lightning send](#lightning-send) entries for how Lightning sends and receives are handled as swaps.

### Cooperative leave

Cooperative leave is the normal way to withdraw Ark funds to a standard on-chain Bitcoin address. You forfeit your VTXOs in an upcoming round and the operator includes a direct on-chain output paying your chosen destination in the batch transaction. It is faster and cheaper than unilateral exit because both you and the operator sign cooperatively. See [Leaving Ark](/concepts/leaving-ark/).

### Esplora

Esplora is a REST API for querying Bitcoin chain data and UTXOs. Wavelength’s `lwwallet` backend uses an Esplora-compatible HTTP endpoint (configured as `walletEsploraUrl`) to watch boarding deposits, fee estimates, and on-chain activity. The endpoint must implement the Esplora `/address/:addr/utxo` API (electrs or Blockstream-compatible) and is set via the `walletEsploraUrl` field on `RuntimeConfig`. See [Networks & config](/concepts/networks-and-config/) and [System architecture](/introduction/system-architecture/).

### Lightning receive

A Lightning receive brings value into your Ark wallet over Lightning through an atomic swap. When you create a Lightning invoice, the swap server and operator set up a vHTLC; once a payer settles the invoice, you claim the locked funds as Ark balance. See [Lightning payments = swaps](/concepts/lightning-payments-are-swaps/) and [Receive a Lightning payment](/guides/receive-a-lightning-payment/).

### Lightning send

A Lightning send moves value out of your Ark wallet over Lightning through an atomic swap. You fund a vHTLC from your Ark balance; the swap server pays the destination invoice and claims the vHTLC once Lightning settlement reveals the preimage. See [Lightning payments = swaps](/concepts/lightning-payments-are-swaps/) and [Send a payment](/guides/send-a-payment/).

### Mailbox

The mailbox is the operator’s asynchronous message channel for events that the wallet cannot pull synchronously over a simple RPC. Swap notifications, round updates, and similar push-style events arrive through the mailbox edge, which shares the Ark operator address (`arkServerAddress`). Wavelength polls and acknowledges mailbox messages as part of its background sync. See [Activity & events](/concepts/activity-and-events/) and [System architecture](/introduction/system-architecture/).

### OOR (out-of-round)

OOR (out-of-round) transfers move VTXOs between owners without waiting for the next batch round. They are the mechanism behind instant off-chain Ark payments and vHTLC funding or claiming. OOR is an advanced topic; Wavelength handles OOR transfers internally and detailed protocol coverage is deferred to a later docs pass. See [Lightning payments = swaps](/concepts/lightning-payments-are-swaps/) for how OOR funds and claims a vHTLC.

### Operator (ASP)

The operator, also called the Ark Service Provider (ASP), runs the Ark coordinator that constructs batch transactions, co-signs off-chain spends, and maintains the VTXO index. You trust the operator for availability and honest signing, but not for custody: you can always reclaim funds on-chain via unilateral exit if the operator stops cooperating. Configure the operator endpoint as `arkServerAddress`. See [System architecture](/introduction/system-architecture/).

### Passkey

A passkey is a phishing-resistant credential (WebAuthn on web, the platform authenticator on mobile) that can protect a Wavelength wallet instead of a password. Wavelength derives the wallet’s encryption from the passkey’s PRF (pseudo-random function) output, so the same passkey deterministically unlocks the same wallet. Passkey support requires platform support; see the requirements pages.

### Round

A round is one cycle in which the operator collects participant requests, constructs a batch transaction, co-signs it with clients, and broadcasts it on-chain. Rounds run on a fixed periodic cadence, aggregating boarding inputs, leave requests, batch swaps, and new VTXO allocations into a single on-chain transaction. Your boarded deposit becomes spendable Ark balance once the round that includes it confirms.

### Transport

A transport is the layer that runs the embedded wallet daemon for a given platform. The Wavelength SDK ships two: the web transport (`wavelength-web`, daemon compiled to WebAssembly, reaches gateways over REST) and the React Native transport (`wavelength-react-native`, daemon compiled into the app binary, uses gRPC). Both implement the same `WavelengthClient` contract, so app code is transport-agnostic.

### Unilateral exit

Unilateral exit is the emergency fallback for withdrawing Ark funds without operator cooperation. You broadcast the pre-signed virtual transaction tree path for your VTXO on-chain and claim it after a CSV timelock, without needing the operator’s signature. It works even if the operator goes offline, but it is slower and more expensive than cooperative leave. See [Leaving Ark](/concepts/leaving-ark/).

### vHTLC

A vHTLC (virtual HTLC) is an Ark output whose taproot script encodes hashlock-and-timelock logic similar to a Lightning HTLC, but settled inside Ark rather than on the Bitcoin base layer. Lightning receives and sends both pivot on a vHTLC: the payer and receiver keys, operator key, payment hash, and timelocks determine who can claim or refund the output cooperatively off-chain or, as a last resort, unilaterally on-chain. See [Lightning payments = swaps](/concepts/lightning-payments-are-swaps/).

### VTXO

A VTXO (Virtual Transaction Output) is an off-chain output that represents spendable bitcoin inside Ark. You hold VTXOs after boarding or receiving a payment; you spend them through cooperative off-chain transfers, batch swaps, cooperative leave, or unilateral exit. Each VTXO is backed by an on-chain batch output and carries a CSV-based expiry, after which you must refresh or exit. See [Balances & VTXOs](/concepts/balances-and-vtxos/).