expansion vs distention

expansion and distention both are nouns.

Word	Noun	Adjective	Verb	Adverb
expansion	Yes	No	No	No
distention	Yes	No	No	No

As nouns, distention is a hyponym of expansion; that is, distention is a word with a more specific, narrower meaning than expansion:

expansion: the act of increasing (something) in size or volume or quantity or scope
distention: the act of expanding by pressure from within

Other hyponyms of expansion include dilatation, dilation, distension, stretching, inflation, magnification, extension.

expansion (noun)	distention (noun)
the act of increasing (something) in size or volume or quantity or scope	the act of expanding by pressure from within
adding information or detail	the state of being stretched beyond normal dimensions
a discussion that provides additional information
a function expressed as a sum or product of terms