It's a range...nothing specific. Low mids are what give things "warmth," - this guitar sounds warm and wonderful solo'd...but if you put a bass track with it, they'd be walking all over each other and you'd have some pretty serious mud in no time.
Anyway, for me, I consider "low mids" to be ABOUT 100hz up to ABOUT 450Hz. At any rate, that seems to be the general area that I usually have to clean up the most.