Some definitions that I've run across:
Expert: "X" is the unknown factor and a "spurt" is a drip under pressure.
Consultant: one who is fifty miles from home and knows all the ways to lose money...by experience.
or
Consultant: one who listens to what you've done, lists the mistakes you've made and identifies why you should have made other decisions and then proceeds to offer alternatives that may, or may not, prove successfull.
or
Consultant: One who's only "stake" in the matter is their fee.
B.S.: knows a little bit about a lot of things.
M.S.: knows a little bit more about a lot less.
PhD.: knows a whole lot about very little.
Alternative:
B.S.: we all know what that means, don't we?
M.S.: more of the same.
PhD.: piled higher and deeper.
Remember, those that can, do.
Those that can't do, teach.
Those who can't teach, teach teachers.
Those that can't teach teachers, consult.

Chef Pierre (aka Pete V. McCracken)