A macro lens is generally a lens that is capable of resolving at least a 1:1 reproduction. It doesn't have anything to do with the focal length of the product. Though some will argue that it's 1:2 or even 1:4.

Macro lenses also allow close (or very very close) focusing distances, something not possible with an equivalent focal length non-macro lens.

When you're looking at different lenses make sure you look at the practical focusing distance because how close you get is going to make a big difference with the magnification. Also, with some subjects you can get super close (within a couple of inches or less) and with others you won't have that opportunity. Here a longer focal length is going to help you. Also of note is the depth of field you'll achieve with different combinations of lenses and accessories. I've played around with some reversed lenses with only a few millimeters of focus. You can make some interesting shots, but even on something as small as a fly, you'd probably only get part of it n focus.