Ark protocol glossary

Here’s a brief reference guide to the terms and concepts used in the Ark protocol. Terminology can vary across different organizations, so bear in mind these are specific to Second’s implementation of the Ark protocol.

Term	Description
Ark server	A central server that coordinates rounds, helps with boarding and off-boarding, deploys liquidity, and enables users to transact over the Lightning Network. The Ark server does not custody any user bitcoin.
Arkoor	Short for “Ark out-of-round”—the method used for Ark payments. Arkoor transactions create spend VTXOs.
Board transaction	The on-chain funding transaction created when a user boards onto Ark. Once confirmed, it serves as the chain anchor for the resulting VTXO.
Boarding	The process of getting bitcoin onto an Ark. A user co-signs a special type of funding transaction with the Ark server and broadcasts it. Once confirmed, the resulting transaction represents the creation of a new VTXO.
Branch	A series of interdependent, off-chain transactions that break up the round transaction into successively smaller chunks until a user’s Ark balance is released. Each branch transaction can only be broadcast after the preceding branch transaction has been broadcast.
Chain anchor	The on-chain transaction output that a VTXO’s validity depends on. For in-round VTXOs this is an output of the round transaction; for boarded VTXOs it is an output of the board transaction. The chain anchor must be confirmed on-chain for the VTXO to be spendable.
clArk	Short for “covenant-less Ark”—an implementation variant that uses recursive multisigs instead of CTV covenants. Each node policy contains a multisig with all the public keys of all the leaves below it, plus the server key.
Connector	A mechanism that ensures forfeit transactions are only valid if a specific on-chain payment is broadcast. Used in on-chain payments and offboards. For rounds, connectors have been replaced by hash-locks.
Delegated refresh	Refreshing VTXOs by having designated co-signers sign on the user’s behalf, designed for mobile devices that cannot reliably wake during scheduled round windows. Requires trusting that at least one designated co-signer behaves honestly.
Exit transaction	A transaction that releases a user’s bitcoin on-chain, at the leaf of a transaction tree. Used when a user wants to retrieve their bitcoin from the Ark protocol.
Lifetime	The time limit set on VTXOs that requires users to spend or refresh their VTXOs before they expire. If a VTXO expires, both the user and the Ark server can unilaterally spend the bitcoin on-chain. VTXO lifetime is configurable by the server operator, but we’re expecting it to be in the region of 30-60 days.
Forfeit transaction	A transaction that allows a user to give up ownership of their VTXO. It uses one of the two spending paths in an exit transaction and grants all bitcoin to the Ark server. Used in refreshes and offboards.
hArk	Short for “Hash-lock Ark”—a protocol enhancement introduced in January 2026 that enables delegated refreshes for mobile devices and streamlines the refresh process.
In-round	A term used for VTXOs that are included in the transaction tree embedded in the round transaction. All refreshed VTXOs are in-round.
Leaf	The final transaction in a branch, releasing a user’s bitcoin on-chain. See exit transaction.
Lightning gateway	A Lightning node connected to the Ark server that enables users to transact with the broader Lightning Network and users on other Ark servers.
Malicious exit	An attempt by a user to broadcast VTXO transactions on-chain (initiate an emergency exit) after forfeiting them. Should always fail due to timelocks securing exits.
Movement	A wallet-level operation that changes the state of one or more VTXOs—such as a round participation, arkoor payment, board, offboard, exit, or Lightning send/receive. A movement is created when the operation starts, locks any affected VTXOs while in progress, and transitions to a final status (successful, failed, or canceled) when the operation completes.
Off-boarding	The standard, cooperative process of withdrawing bitcoin from Ark. A user forfeits one or more VTXOs and receives an output for the same amount on-chain.
Refresh	The process of forfeiting old VTXOs for new ones during an Ark round, before their lifetime ends. The wallet app participates interactively during the round, signing its own exit branches in the transaction tree. See also delegated refresh for mobile devices that cannot reliably wake during scheduled rounds.
Root	The single on-chain transaction that all associated branch and leaf transactions emanate from. For in-round VTXOs, this is the round transaction; for boarded VTXOs, it is the board transaction. See chain anchor.
Round	A periodic event initiated by the Ark server where users can refresh their VTXOs. Only users participating in the refresh have their VTXOs included in the round transaction.
Round transaction	The root transaction of a transaction tree that is broadcast on-chain during an Ark round. This transaction provides users with verifiable assurance that they can unilaterally retrieve their bitcoin on-chain.
Sweep	The process by which the Ark server transfers all forfeited bitcoin from an expired round to its operational wallet. Usually requires only a single on-chain transaction, unless a user in the round has initiated a partial or full emergency exit.
Transaction tree	A hierarchical structure of transactions that enables UTXO-sharing. It consists of a root transaction (broadcast on-chain), branch transactions, and leaf transactions, allowing multiple users to share a single on-chain UTXO while maintaining control of their bitcoin.
Transaction tree depth	The number of transactions a user must broadcast to complete an emergency exit. For a refresh VTXO, this is determined by the number of branch layers in the tree, which scales logarithmically with the number of users in the round. Each spend VTXO in a chain adds to the exit length.
Emergency exit	The non-standard, non-cooperative process of withdrawing bitcoin from Ark. The user independently broadcasts each off-chain transaction from their VTXO in sequence, until the bitcoin is delivered to an address they solely control. The availability of emergency exits guarantees that users maintain self-custody of their bitcoin. Sometimes shortened to “exit”.
VTXO	Short for “virtual unspent transaction output” or “virtual UTXO”. A series of off-chain, pre-signed transactions that a user can broadcast at any time to retrieve their bitcoin on-chain in an emergency. VTXOs have a limited lifetime and must be refreshed periodically.