An exercise is a movement performed to enhance or maintains physical fitness. For our purpose, we focus only on the common characteristics of an exercise and treat them as the parent for all that follows, i.e. exercise variations and resistance exercises based on exercise variations.
One such example is the squat. Any variation of the squat achieves ankle (dorsiflexion), knee, and hip flexion. Variations of the squat will add other characteristics like hip abduction, hip rotation, staggered stance, etc.