Link

A link is a connection between 2 cells or candidates.

For 2 cells, the connection is caused by the fact the both cells are peers. They are located in the same house.

2 candidates are connected when they belong to the same cell, or when they represent the same digit in a single house. Their respective cells are peers in this case.

Links are used in chains and loops. The way they are used in a chain or loop differs for strong links and weak links.