It goes by several names.  Mission Statement, Purpose Statement,  Core Purpose,  and often it is mistakenly referred to as the Brand Promise.

Avoid the Dilbert Definition (a long paragraph written by the executive team, using large words meant to confusion everyone in the company about what they should be doing) and keep it simple and straightforward.  Get it down to a sentence if you can, using simple easy to understand words.

It should say what the key thing you do, make reference to who you do it for (your customers) and why you do it.

One of the best ones I’ve ever seen is one that one of my clients uses, and I can’t take any credit for it.  It’s a vending machine company.

“We bring refreshment to the weary workforce”

It’s clear, it tells the team exactly why the company exists, and exudes a bit of nobility that gives the team something meaningful to accomplish, something to get passionate about.

It motivates the team, gives them direction, and is built on the internal attributes of the organization.

It’s internally focused.

And he never mentions it to his customers, just his team.

How does your Core Purpose read?  Does it define what you do?  Does it inspire the passion with which you do it?  Does it identify who you do it for?  Is it less than two sentences long?