The archetypal concept of soul mates is timeless and has probably existed for as long as humanity has… However, the one individual most rightly credited with the concept of soul mates in the Western World is the ancient Greek philosopher, Plato (427 BC – 348/347 BC)... According to Plato's creation story as contained in the Symposium: In the beginning there was only one human. And this human was "created perfection."
The rather odd thing was that this first "created perfection" human had all the "necessary" male parts, as well as having all the "necessary" female parts (if you get my drift). However, this perfect human (with all of the "necessary" parts) was then tragically split apart – and, since that time, the two halves (the male parts and the female parts) have been forever searching for one another in order to join together and regain their sense of original created wholeness.
"Each of us when separated, having one side only... is but indenture of a man, and he is always looking for his other half." Plato from the Symposium
That, my friend, is the bona fide traditional Western tradition behind the concept of soul mates… two halves looking to be made whole… (and yes, I know that's not a popular view of "healthy relationships" these days.)
And yes, before you write in attempting to educate me on the matter, I realize there are several rather popular "New Age" theories (such as twin souls and twin flames) floating around out there that are much more complex in nature and allegedly more ancient.
Definitions
Here are a couple of more current definitions of soul mates that I especially like and find useful.
"A soul mate is someone to whom we feel profoundly connected, as though the communicating and communing that take place between us were not the product of intentional efforts, but rather a divine grace. This kind of relationship is so important to the soul that many have said there is nothing more precious in life." Thomas Moore Soul Mates And...
"As in a spiritual tradition, we are called on in the new 'tradition of two' to do 'soulwork' - to expand our souls, heal our past wounds, and evolve as humans... How do you know when you're with a soul mate? It's not a matter of magic. You know you're with a soul mate when you are both doing your soulwork...
Unfortunately, many potential soulmates get lost because they don't know how to do their soulwork together. They may linger in the popular definition, where upset feelings shouldn't happen, where upset is a bad sign. This explains why so many people feel they have been with a soul mate - but then, somehow, lost them." John and Bonney Grey, Becoming Soulmates
Using these definitions, it definitely sounds like one might actually encounter more than one soul mate connection during a single lifetime? We often do...