Definition of "guppy"

A tiny freshwater fish, Poecilia reticulata, popular in home aquariums, that usually has a plain body and black or dark blue tail for the females and a more colorful tail for the males.  examples

(by extension) Any tiny fish.  examples

(paintball) A tube holding paintballs before they are loaded into the gun.  quotations  examples

(climbing) A hold across the palm of the hand rather than the fingertips.  examples