Why is called the butterfly effect?

Rating: 8/10 (8 December 2005)

So why is called the butterfly effect? The only part I see something related to a butterfly is the image first showed of a brain.
...explanation...?
Also what causes these effects in the first place and why?
What difference did it make when he scared off Kayleigh?
How did Tommy become a mister nice guy after he save the lady and her baby?
...he was the one with the stupid idea to put the explosive into the stranger's mailbox
...duh...
but I gotta admit, this movie has a lot of good actors.
They really impressed me, although some were quite not really convincing, not gonna name any names but overall... this is a very good movie. ("very")