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 spending on Ark. Arkoor VTXOs can be created at any time (outside of periodic rounds) and require signatures from both the sender and the Ark server. Arkoor VTXOs still support unilateral exit, but rely on a trust model that assumes the sender and Ark server do not collude. Users can refresh their arkoor VTXOs in any subsequent round to achieve trustless security.
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.
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 a forfeit transaction is only valid if the next round exists, making swaps atomic and safe for clients.
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.
Expiry time	A time limit set on VTXOs that requires users to spend or refresh their VTXOs before expiration. If a VTXO expires, the Ark server can technically claim the bitcoin. Expiry times are configurable by the server operator, but we're expecting them 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.
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 a unilateral exit) after forfeiting them. Should always fail due to timelocks securing exits.
Off-boarding	The standard, cooperative process of withdrawing bitcoin from Ark. Off-boarding is an atomic process handled during rounds, when 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 the expiry time is reached.
Root	The single transaction that all associated branch and leaf transactions emanate from. See round transaction.
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.
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 unilateral 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 a unilateral 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.
Unilateral 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 unilateral 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 an expiry time and must be refreshed periodically.