Preexistence of Christ
Preexistence (also "pre-existence") means "before existence". Literally "preexistence of Christ" means "before existence of Christ—before Christ existed". As applied to Christ, the term has its root in an unconscious materialism, the belief that only matter exists, and accordingly most Christian theologians and apologists use the term to mean instead "Pre-Incarnation of Christ".
The heresy of Arianism teaches that there was a time when Christ did not exist, therefore the term as applied to Christ in its most literal meaning is regarded by mainstream Christian denominations and Churches as heretical and improper. According to orthodox Christian doctrine there never was a time when Christ the Son of God eternally begotten of the Father did not exist. See Athanasian Creed and Ecumenical council.
The ubiquitous imprecision of ignorantly using the contradictory expression "preexistence of Christ" to actually mean "pre-incarnate existence of Christ" has become a deeply-intrenched, unthinking abuse of language, unlikely to change, an ignorant deformity of usage which has become a standard expression to denote the eternity and divinity of Christ as one with God the creator and ruler of the universe before "the word became flesh" (John 1:1-2, 14).