A trellis is a graph

Graph (mathematics)

In mathematics, a graph is an abstract representation of a set of objects where some pairs of the objects are connected by links. The interconnected objects are represented by mathematical abstractions called vertices, and the links that connect some pairs of vertices are called edges...

 of which the nodes are ordered into vertical slices (time) and each node at each time is connected to at least one node at an earlier and at least one node at a later time. The earliest and latest times in the trellis have only one node.

Trellises are used in encoders and decoders for communication theory

Communication theory

Communication theory is a field of information and mathematics that studies the technical process of information and the human process of human communication.- History :- Origins :...

 and encryption

Encryption

In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...

. They are also the central datatype used in Baum-Welch algorithm

Baum-Welch algorithm

In electrical engineering, computer science, statistical computing and bioinformatics, the Baum–Welch algorithm is used to find the unknown parameters of a hidden Markov model . It makes use of the forward-backward algorithm and is named for Leonard E. Baum and Lloyd R...

 for Hidden Markov Models.