What exactly is vision?

From Built to Last: Successful Habits of Visionary Companies :

A well-conceived vision consists of two major components - core ideology and an envisioned future. Notice the direct parallel to the fundamental “persevere the core/stimulate progress” dynamic. A good vision builds on the interplay between these two complementary yin-and-yang forces: it defines “what we stand for and why we exist” that does not change (the core ideology) and sets forth “what we aspire to become, to achieve, to create” that will require significant change and progress to attain (the envisioned future).

Regarding the definition of core ideology:

Any effective vision must embody the core ideology of the organization, which in turn consists of two distinct sub-components: core values and core purpose.

Personally, love this quote about purpose:

Purpose is the organization’s fundamental reason for existence, which like a star on the horizon can never be reached; it guides and inspires forever.

If you’re interested articulating your own purpose, I recently wrote a book - How to Craft an Inspiring Purpose Statement: A Step-by-step Guide for Entrepreneurs. It’s free for a limited time and I think you’ll find it helpful.