Square Link (スクエアリンク Sukuea Rinku) is an unofficial term for several cards that appear in Booster 1: Encounter with the Parallel Worlds.
Square Link consist of two components: a Z/X with automatic ability that triggered when the card appear in Square (with a single exception), and a vanilla Z/X required by said ability, all of same color. Also, at least one of the linked cards has an Ignition Icon. All linked card's illustration are drawn by the same illustrator and can be connected to form a single image.
This kind of interaction is later expanded in Booster 2: Roar of the Titans with several cards like "Demonic Wolf, Fenrir", "Looting Machine, Plunder", "Lawbringer, Carstensz", and "Dog Girl, Were-Dog" that interacts with specific Z/X in Square in a different style.