Flank

The creatures on the far right and far left of a player’s battleline are on the flanks of the line. A creature in this position is “on the flank” and is referred to as a “flank creature.” Any time a creature enters play or changes control, the active player chooses which flank of its controller’s battleline it is placed on.

If a battleline only has one creature in it, that creature is on both the left and right flank and is considered a flank creature.