Real Dictionary

pop up
phrasal verb: If someone or something pops up, they appear in a place or situation unexpectedly.

pop up in American English
to hit a pop fly

pop-up
A pop-up book, usually a children's book, has pictures that stand up when you open the pages.
A pop-up toaster has a mechanism that pushes slices of bread up when they are toasted.
On a computer screen, a pop-up menu or advertisement is a small window containing a menu or advertisement that appears on the screen when you perform a particular operation.

pop-up in American English
of or designating a book designed so that cut-out portions of an illustration lift when the book is opened to its page, producing a three-dimensional effect
pop1 (sense 5)
adjective: having pieces of artwork fastened to the pages so that when the page is opened, a three-dimensional cutout or object is formed and, sometimes, movement of a picture element, such as a door opening, can be activated by pulling a tab
noun: a pop-up book

pop-up in British English
adjective: (of an appliance) characterized by or having a mechanism that pops up
noun: something that appears over or above the open window on a computer screen
verb: to appear suddenly from below

pop-up in Retail
adjective: Pop-up is used to refer to stores and other small businesses that opened temporarily to take advantage of a trend or a seasonal or short-lived product, closing down when the selling opportunity has past.