Abbreviation of トレーニング, used as a suffix in compounds like 筋トレ (muscle training) or イメトレ (image training). Not used as a standalone noun.
I do muscle training every morning.
I never skip image training before a match.
home workout; working out at home
working out at home; home workout
strength training; resistance training; muscle-building
Clipping of トレーニング, from English 'training'.