I think the idea of having one life purpose is misleading or inaccurate for many people. If we're meant to evolve and learn throughout our lives, I think we can have different kinds of purposes and different sense of goals during different life phases.
I like Mark Lapierre's definitions, but I'd expand. I'd describe a purpose as a key reason why we feel we exist. I don't necessarily see a purpose as continual. People experience revelations about a sense of purpose and may clarify or change it throughout their lives. We associate purpose with components of an identity; perhaps our personal life (partner, family, children); a career where we give back; a spiritual connection to something beyond us. Some of our purposes appear ephemeral, others appear more enduring. Some are purely individual and others would also be considered collective, such as universal consciousness.
Consider Rick Warren and his questions in his book, "The Purpose-Driven Life." Although William Shakespeare states "The devil can cite Scripture for his purpose," the God described within the pages of the Bible might see its purpose as teaching lessons which may or may not relate to something evil. The intention and result of a purpose then, can vary as well.
"Myth is an attempt to narrate a whole human experience, of which the purpose is too deep, going too deep in the blood and soul, for mental explanation or description.” -D.H. Lawrence
