What is a pop-up ball in a baseball game?

I looked it up on line and the wikipedia says it is

a kind of "high fly ball." But another website says

it is a kind of ball a pitcher pitch. It's the opposite

of "sinker."

Which one is more correct? Or can it mean both?

Thank you very much for your help!
I'm not a rabid fan, but I've never heard it applied to a pitch. But there are many, many expressions used to describe "specialty" pitches.

I tend to think of the expression as describing flies on which the umpire calls the "infield fly rule," but I guess it also applies to "pop fouls."

Edit. I did have the following pitch in the back of my mind as a possible candidate, but couldn't think of the name. I quote: The rising fastball is a pitch where the ball appears to hop up as much as a third of a meter with a sudden increase in speed.

The infield fly rule is too restrictive, relating also to the number of outs. Let's say, a pop-up is a fly ball which is likely to be in the air for at least three or four seconds and not travel more that a hundred feet or so from the plate - that is, not likely to reach into the outfield by a significant amount.
It goes high, but not far.
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Thank you, Avangi and Grammar Geek...^^
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