I found a few useful definitions of mission statement on Google.
A few that I liked:
"A philosophical or value statement that seeks to respond to the "why" of the organization's existence, its basic reason for being. Mission statement is not defined in expressions of goals or objectives, rather it reflects a realistic but farsighted determination of who the organization is, who it serves, what it does, and what it can accomplish."
"A general statement of a vision in word form. It is important to have a rich representation of the vision in all the senses. Then the mission statement can be written in language which allows all parties to it to derive meaning from it, yet be precise enough to guide them towards achieving it."
"A statement of the role, or purpose, by which an organization intends to serve its stakeholders. Describes what the organization does (current capabilities), who it serves (stakeholders), and what makes the organization unique (justification for existence)."