completion vs ending

completion and ending both are nouns.

Word	Noun	Adjective	Verb	Adverb
completion	Yes	No	No	No
ending	Yes	No	No	No

As nouns, ending is a hypernym of completion; that is, ending is a word with a broader meaning than completion:

Other hypernyms of completion include conclusion, termination.

completion (noun)	ending (noun)
a concluding action	the act of ending something
(American football) a successful forward pass in football	the end of a word (a suffix or inflectional ending or final morpheme)
	the last section of a communication
	event whose occurrence ends something
	the point in time at which something ends