Those terms are broad and open to interpretation. For a bass guitar, you might call the range from 120Hz to 400Hz "low mids", 400Hz to 900Hz "mids", and 1KHz to 2KHz "high mids", but I wouldn't be surprised or bothered if someone else picked different numbers.
If I had to pick a number for each, I would consider 230 hz low mids, 800 mids, and 1500 hi mids...which would place them within the above ranges.