Flank
From Archon Arcana - The KeyForge Wiki
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.
Related Topic: Battleline
—KeyForge Master Rulebook v17.3 March 2024
Commentary
If there is a single card in a battleline, it is both in the Center of the Battleline and on both flanks.
Source: Flank•Center of the Battleline