Break (ブレイク Bureiku) is a term that refer to the act of turning a Z/X into another Z/X from a specific zone, in which the former Z/X will become a face-down Force under the latter. If that Z/X is currently being attacked, the attack target will automatically changed to the new Z/X.
It also refer to the state of Z/X with face-down Force associated to it, most often because of the aforementioned act, and written as "Break State" (ブレイクしている Bureiku shiteiru) in the card text.
1018.1 "Break" refers to the state of card with face-down Force associated to that card.
1018.1a If the amount of face-down Force associated to a Break state card became 0 due to some ability, that card is no longer in Break state.
1018.2 If there is an instruction to "one of (card A) on Square Breaks and turns into (card B) from [text]", the designated player choose a card A on Square, choose a card B from the designated zone, and reveal it if the card is in a hidden zone. Move the chosen card B to the Square of card A, move card A face-down to Force Zone as Force associated to card B, and card B appears in Reboot State. At this moment, if card A is currently in battle where it was attacked, change the attack target of that battle to card B, and continue the battle.
1018.2a If you are unable to perform the "Break" instruction for any reason, that Break is not performed, then go back to the game state just before the Break is performed.
1018.3 If a Force is face-down, that Force loses its ability.