What is classed as a movable lump?

Could anyone tell me what a movable lump in the breast is classed as please? Should it completely move around or does it sometimes just wiggle a little? I'm really struggling to work out if my lump is movable. I wiggles a little if I rub it but I wouldn't say it's moving freely? It is quite difficult to move but feels like it moves with my finger under the skin?