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Contestation Period

The time window between closing the head and the last time (deadline) a contest is possible by a head participant.

Hydra Head

A running instance of the hydra head protocol. See a reference in essential cardano for more details.

Hydra Network

One or more hydra nodes inter-connected.

Hydra Node

Essential building block and executable piece of software that is used to form a Hydra network.


Reuses the existing smart contract system and code for offchain computations. This guarantees that onchain and offchain semantics coincide, leading to significant engineering simplifications.

Layer 2

Layer 2 refers to a secondary framework or protocol that is built on top of an existing blockchain system. See this page for more details.

Participation Token

An on-chain token that entitles a party to participate in the Hydra head associated with the token.


A participant or logical member of a Hydra head, identified by a pair of verification keys one for the Hydra network and another one for the Cardano network.


A member of a Hydra network, identified by an IP address and a TCP port.

Fuel UTxO

UTxO pieces used to pay for Hydra head transaction fees happening on the base layer (i.e. the Cardano network).

Reference script

See this page for more details.


The manifestation of a chain switch on the base layer, causing the recent history to change. It is a multi-step process between the head and L1. See this page for more details.

Signing key

See a reference in cardano glossary for more details.


Some state of a Hydra head; When multi-signed by all parties, a snapshot is said "confirmed".

State Channel

A common kind of layer two solution. See this page for more details.

State Thread Token

An on-chain token representing the state of a Hydra head.

Verification Key

See a reference in cardano glossary for more details.