As always, here is a little idea I've got. As Wikipedia propose more a breakdown of different animation techniques and just one definition of animation (the last one presented here), I've decided to put some others viewpoints together. Fell free to propose any definition you've read in the comments section, so that I can add them if they are relevant.
From Ollie Johnston and Frank Thomas :
-Illusion of Life : Disney Animation, Preface (Book Link)
Animation is the Illusion of Life :
"[...] Animation that produces drawings that appear to think and make decisions and act of their own volition; it is what created the illusion of life."
From an art standpoint :
-Timing For Animation, Second Edition, Foreword by John Lasseter p.ix (Book Link)
"Time is the essence of animation. It is what makes animation different to other visual arts where the observer controls how much time a piece of art is viewed"
From an art standpoint in relation to contrast :
"Generally animation is made off of two different things; it's a contrast of motion again stillness. The motion is pretty interesting, but the stillness is where the story is told."
-Keith Lango VTS 03 3:10 (Video Link)
From a technical standpoint (Read more on subject on Wikipedia)
-DJ Nicke (Video Link)
"Animation is the illusion of movement through the persistence of vision."
Et Voilà !