That's an interesting question ...
One definition might be "an animated effect that introduces a slide on
screen."
So, when you click from one slide to the next in a slideshow (or even
beginning with the first slide in a slideshow), you can add a brief "opening
animation" when the slide appears.
Here's an example (for PowerPoint 2007):
1. Create a slideshow with two slides. (Put whatever content you like on the
slides.)
2. Go to the second slide in Normal View.
3. On the Animations tab in the ribbon, in the Transitions to This Slide
group, open the Transitions gallery (it's the arrow to the right of the
window with all the transitions).
4. In the Transitions gallery, under Fades and Dissolves, select Fade
through Black(second option from the left).
Note: You might want to set the transition speed to slow so that you can see
it. In the Transition to This Slide group, in the Transition Speed menu,
select Slow.
5. Now start your slideshow. (Press F5.)
6. You'll notice that when you advance the slideshow from the first slide to
the second, that the second slide "fades in from black" when it enters.
That's a transition.
If you're curious about transitions, play around with the different
transitions in that Transitions gallery. You'll find some pretty cool effects
there.
I hope this answers your question!
