"I'd like two butterflies..."
You want a side with that?
Uh... sure.... I'll take the propagative effects with extra ketchup

Usually, a side effect is what you get when you do something, then let it interact with the rest of the world.

Take for example a butterfly, flapping its wings. The side effects of this may well be a hurricane on the other side of the world.

A side effect of eggs are chickens, and a side effect of babies are people. That is, side effects are all relative - they change as you change your point of view. From one view, the side effect of internal combustion engines is motion. From the other, the side effect is exhaust.