The Seder step where the middle matzah is broken is Yachatz.
Yachatz is the fourth of the Seder’s traditional 15 stages. Three matzot are commonly placed on the table, and the middle one is broken. The larger piece is set aside and later eaten as the afikoman, while the smaller piece remains with the other matzot for the meal’s blessings.
The name Yachatz means “halving” or “dividing.” It is performed before Maggid, the section devoted to recounting the Exodus. The act helps make the matzah part of the Seder’s narrative: it is both the bread of affliction and the bread associated with freedom.
A common mix-up is confusing Yachatz with Tzafun. Yachatz is when the matzah is broken and hidden; Tzafun is the later step when the hidden afikoman is eaten.