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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.

Marker Datum​

Its a datum used for the sole purpose of identifying a UTxO.

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.