I believe the term is used as a metaphor for the essence of a person—that unknowable collaboration between mind and body and life. It’s about the essence of a person’s life, all their experiences, their memories, their actions, and their thoughts about all of this. It suggests a direction that we perceive in other people.
Some people we call “old souls.” That usually suggests they have a wisdom that seems to be beyond their years. Where could it have come from?
There are soul-mates; people who connect at such a deep level, it seems like they are perfectly matched in their essential natures.
There are many kinds of soul systems. For example, this author makes up two types:
1) jiva-atma – the individual soul, known as the living entity
2) param-atma – the Supersoul, known as the Supreme Lord, who resides in the hearts of all living entities as the witness.
I googled types of souls and found seven soul systems and four soul systems, “three soul systems” (http://www.kabbalaonline.net/Safedteachings/gor/Three_Types_of_Soul_(71).asp) and who knows what else. There are alien souls inhabiting the bodies of humans and there are souls in inanimate as well as living beings, including animals.
For one, souls are personality systems:
The names reflect the natural proclivities of each soul type:
Servers are naturally accommodating, caring, nurturing, hospitable, altruistic.
Artisans are naturally creative, inventive, imaginative, sensitive, dexterous.
Warriors are naturally forceful, loyal, protective, determined, steadfast.
Scholars are naturally curious, studious, academic, analytical, neutral.
Sages are naturally engaging, articulate, charming, entertaining, expressive.
Priests are naturally inspirational, uplifting, motivating, energising, visionary.
Kings are naturally commanding, assured, powerful, authoritative, decisive.
It think that all of these systems are trying to capture the notion of the true essence of people and things. Soul is the metaphor for that which can’t really be encapsulated. We call it a soul and leave it at that as if we have explained it.
In truth, it is extremely difficult to explain the essence of a being. “Soul” is a useful metaphor, even for atheists. Most people understand intuitively what is meant. The atheists who get annoyed with the term (I used to be one of those) may eventually learn to lighten up and use the word without religious connotations. It is possible to do that.