Force (フォース Fōsu) is a term for cards that connected to a Z/X by being placed face-up under that Z/X on Square.
Force is mainly created by the Alter Force ability, but there's also other cards that able to put a card under a Z/X as a Force as well. While the Force itself is located directly under the related Z/X on that Z/X Square, it is treated to be located in the Force Zone under the rule.
Card abilities or effects may refer to a Force possessed by a Z/X. If an ability or effect refers to a Force that was put into a different zone, it refers to that Force's current information in that zone. If it is put into a hidden zone or becomes face-down in that zone, then it refers to the information before the Force was put into that zone instead.
If a Z/X with Force leaves the Square, the Force is put into Trash per Rule Effect.
Turning Card into Force
Cards can be turned into the Force of another card by ability or effect that states "...put (...) under (...) as Force" or "...turn (...) into (...) Force (...)" such as the aforementioned Alter Force ability. Most of them will state whether the Force is put in face-up or face-down position, although the Force will be automatically put as face-up if it's not stated in the text. A face-down Force will lose all of its ability that it has if it were face-up.
Other than card ability and effect, the Break keyword action is also able to turn a Z/X into the Force of another Z/X.