The baseball team members know that when the pitcher wipes his brow, he is about to throw a fastball. the pitcher's brow-wiping is a

The pitcher's brow-wiping is a symbol.
A symbol is an idea, a word, an action, which is supposed to represent something more than it actually does. In this case, when the pitcher wipes his brow, he's not simply wiping his brow for the sake of it, but he is signaling his teammates that he is about to throw a fastball and that they should be ready for it.