expansion vs distension

expansion and distension both are nouns.

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

As nouns, distension is a hyponym of expansion; that is, distension 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
distension: the act of expanding by pressure from within

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

expansion (noun)	distension (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