inflexion vs declension

inflexion and declension both are nouns.

Word	Noun	Adjective	Verb	Adverb
inflexion	Yes	No	No	No
declension	Yes	No	No	No

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

inflexion: a change in the form of a word (usually by adding a suffix) to indicate a change in its grammatical function
declension: the inflection of nouns and pronouns and adjectives in Indo-European languages

Other hyponyms of inflexion include conjugation, paradigm, pluralisation, pluralization.

inflexion (noun)	declension (noun)
a change in the form of a word (usually by adding a suffix) to indicate a change in its grammatical function	a class of nouns or pronouns or adjectives in Indo-European languages having the same (or very similar) inflectional forms
	a downward slope or bend
	process of changing to an inferior state
	the inflection of nouns and pronouns and adjectives in Indo-European languages