|Definition - Any tense that denotes a past action that is still in progress.|
(1) When I walked to the store, I was thinking of you.
(2) Every Tuesday I used to eat hot dogs.
Oxford English Dictionary - Its first citation is from 1530:
"The preter imperfit tens, as je parloye."
(Jehan Palsgrave, Lesclarcissement de la langue françoyse, 84)