Submission and what it means varies from person to person. For every submissive there will likely be a different definition. That is one of the first and foremost stumbling blocks of our communities ability to find cohesion and organization to get momentum in the same manner the LGBT community did.
With that in mind, my definition of a submissive and what makes a submissive: A submissive finds completion in their submission, where a bottom finds enjoyment from a scene. A submissive will often times be a type 'A' personality, and find comfort and release in their submission. Realizing an inner peace that they must sacrifice in order to do for others what they see needs done. Many times a submissive is submissive to only one, and to everyone else is the alpha personality. Other times a submissive will be that person that has great trouble being in the lead of any group, or making any decisions without some sort of previously laid out structure to follow.
Slaves, while holding many of the same personality traits as submissives, have a different mindset than submissives do. A slave will sacrifice their own well being, their own interests for the one they have devoted themselves to. Irregardless if that devotion is properly placed or not.