They mean the same thing, though "make a pass at" has a slightly stronger sexual connotation, suggesting that someone actually tried to get someone to engage in a sexual activity, be it simply kissing or more.
I don't know how to quote from two different posts in this one reply. First, I just wanted to say Corum notes a 2nd meaning of 'hit on' (= attempt to get something, typically money, from someone), but I want to stick to the other meaning of 'making advances towards' and look at how this differs from 'make a pass at'.
Here are the definitions from the Oxford English Dictionary:
hit on = (AmEng, informal) make sexual advances towards
pass = (informal) an
amorous or sexual advance towards someone
So the dictionary definition and my understanding are the exact opposite of Admiral's - it is 'hit on' that has a strict sexual connotation, whereas you can 'make a pass at' someone for more honourable reasons. Could this be a difference in usage between AmEng and BrEng?
OK, sticking to my understanding of the differences:
hit on - make an often persistent (eg throughout the duration of a party) and obvious attempt to persuade someone to come to bed with you (or to
'get off with someone' or as my circle of friends say rather crudely (including women, I should add)
'get into someone's pants'). This would usually involve
chatting the other person
up, but buying them drinks, standing up close, slyly touching without asking and being intimate on the dance-floor are all ways of 'hitting on' someone. If it works, you
got lucky; if it doesn't you're a
creep (at least if you're a man).
make a pass at someone - there's no sense of persistency with this and also you would usually make a pass at someone by saying something. And yes, sex would often be on the mind, but it need not be - you might just be looking for love and a long-term relationship.
Now, take me, in practice. In my student days, I might occasionally have hit on a girl, although I would never have used that expression since I'm English and 'hit on' wasn't used in BrEng then (even now, I think it's mostly AmEng, though I haven't discussed this with any of the skateboarding emos in my neighbourhood). Instead, I would have said to a friend at a party (because, come to think of it, I'd usually be the one following events): 'you were
coming on to her [strong/big time/heavy]'; 'you were [
really] giving it a go with her'; 'you were
[really] trying it on with her'; or something such.
Today, though, I would only ever 'make a pass' at a woman. Take my son's school. There's a single mum there. Two years and counting, but someday I might ask her out on a date, and, hand on heart, I'll have romance on my mind. Now, if I ever do follow through, that will definitely be a pass, not a hit ('hit' - ugh! The thought makes me cringe).