stretching vs physical exercise

stretching and physical exercise both are nouns.

Word	Noun	Adjective	Verb	Adverb
stretching	Yes	No	No	No
physical exercise	Yes	No	No	No

As nouns, physical exercise is a hypernym of stretching; that is, physical exercise is a word with a broader meaning than stretching:

stretching: exercise designed to extend the limbs and muscles to their full extent
physical exercise: the activity of exerting your muscles in various ways to keep fit

Other hypernyms of stretching include exercise, exercising, physical exertion, workout.

stretching (noun)	physical exercise (noun)
exercise designed to extend the limbs and muscles to their full extent	the activity of exerting your muscles in various ways to keep fit
act of expanding by lengthening or widening