propensity vs tendency

propensity and tendency both are nouns.

Word	Noun	Adjective	Verb	Adverb
propensity	Yes	No	No	No
tendency	Yes	No	No	No

As nouns, tendency is a hypernym of propensity; that is, tendency is a word with a broader meaning than propensity:

propensity: a natural inclination
tendency: an attitude of mind especially one that favors one alternative over others

Other hypernyms of propensity include disposition, inclination.

As nouns, propensity and tendency are synonyms defined as:

Other synonyms of propensity include leaning.

propensity (noun)	tendency (noun)
a disposition to behave in a certain way	a characteristic likelihood of or natural disposition toward a certain condition or character or effect
a natural inclination	an attitude of mind especially one that favors one alternative over others
an inclination to do something	an inclination to do something
	a general direction in which something tends to move