Terminology

Basic terminology
Following is a list of the terminology required to explain the rules of the game.

Time
Time is a non-physical, discrete dimension which always flows forward as moves are made on the boards. Its unit is the turn. The time dimension is usually abbreviated with " ".

Timeline
The timeline dimension is a non-physical, discrete dimension which represents the alternate history of the boards. The timeline dimension is usually abbreviated with " ".

The timeline dimension's unit is called "a timeline" and is abbreviated with " ". 1 timeline is equivalent to 1 square.

New timelines are created whenever a piece is moved to an already-moved board. Timelines appear on the moving player's side, with white's timelines being indexed with positive integers (+1, +2, etc.) and black's timelines being indexed with negative integers (-1, -2, etc.) This indexing system is used to denote a board's Timeline Index.

File
Alternate name for the  physical dimension, its unit is the square.

Rank
Alternate name for the  physical dimension, its unit is the square.

Turn
Synonyms: Step, Time step

Unit of time, abbreviated with " ". One turn corresponds to two half-turns or a succession of white and black playing. 1 turn is equivalent to 1 square, but since there are two half-turns in one turn, diagonals are stretched horizontally when all of the boards are visible.

"White's turn" and "Black's turn" may also be used to refer to either player's half-turn.

Half-turn
Synonyms: Sub-turn, Board color, Half-Step

A half-turn corresponds to one board in time. Half-turns alternate between white's turn/boards to play (on) and black's turn/boards to play (on). White cannot interact with any board that are on black's half-turn and vice versa.

White's half-turn is usually denoted with the letter " " and black's half-turn with " ".

Move
A movement of one piece, or castling.

Spatial move
Synonyms: Physical move

A spatial move is a move that stays within the boundaries of one board. In other words, spatial moves are moves that can be done in 2D Chess. With a spatial move, the source and target boards are the same and only one new board is created by it.

Spatial moves never result in a new timeline being created, but can be used to perform a time travel from another board.

Non-spatial move
Synonyms: Super-physical move

A non-spatial move is a move that does not stay within the boundaries of one board. They cannot be done in 2D Chess and rely on the time and  timeline dimensions.

With a non-spatial move, the source and target boards are different and two new boards are created by it.

A non-spatial move may result in a new timeline being created if the target board was already played on.

Moveset
Synonyms: Action, Move sequence

A collection of moves by a single player which advance the present to the opponent. The game consists of alternating players submitting movesets.

Time travel
A time travel is a non-spatial move to an already-moved board, which results in a new timeline being created. The new timeline will contain the moved piece and will be placed on the moving player's side.

It is usually shortened to.

Present
The time of the earliest active board/timeline.

Inactive Timeline
A timeline is inactive if it does not affect the advancement of the present. If one player has created more than one more timeline than their opponent, the excess timelines are inactive until the opponent makes more timelines.

Active Timeline
The opposite of an inactive timeline; one which does affect the present.

Check
If passing on a timeline allows the opponent to attack any of the moving player’s kings, that timeline is in check. A player is in check if any of their active present timelines are in check.

Checkmate
If a player is in check and has no legal moves, they are in checkmate.

Softmate
"check on one board" - a timeline in softmate is one which contains an piece giving check in a way that cannot be avoided solely by moving pieces from that timeline. Often this requires a piece from an adjacent timeline to jump back in time, then either capture the piece giving check or win before the present catches up.

Hardmate
Inescapable checkmate on one timeline. Usually caused by 2 different pieces giving check to 2 different kings (or 2 historical kings). In this situation, the opponent's only hope is to jump back in time and try to win or draw before the present catches up with the hardmate.