Dominant male.
If I ever used the word in real life I'd mean this, but in erotica it seems to be a different sort of thing. A lot of M/F work has some of this flavoring, not necessarily degrading women so much as being really preoccupied with how powerful the man in the story is.I was under the impression that "alpha male" was a code word for a seriously misogynistic swathe of current western culture.
Alpha males and BDSM are two different things, even if there is sometimes overlap. At least I've gotten the impression that they're hitting two very different kinks. BDSM involves dominance and submission in a lot of sexual contexts, whereas the alpha male stories I've read are about dominance and submission in a broader context.
An alpha male is in contrast to a beta male. Lots of warm and fuzzy romance stories feature a beta male, who is a family man, a trusted VP, or a best friend rather than a hyper-masculine alpha male.
There's a good discussion of it here: http://viviennecourtoise.blogspot.ca/p/alpha-male-vs-beta-male.html
Uh, bit of a disclaimer on that link. It's completely fine as a way of framing romance or thinking about men, but the alpha/beta/etc male terminology has been pretty thoroughly taken over by pick up artists and other misogynist types. It might have unpleasant connotations for some people.