The story
From A-movement, with the prime symbol (′), also read as "bar" (see X-bar theory), denoting a non-argument position. Named such because A′-movement targets positions that are not A-positions.
Etymology from Wiktionary, CC BY-SA 4.0.
A′-movement
Today it means: Movement of a constituent to a non-argument position, such as a specifier, as in topicalization or wh-movement. Full definition →