What is "unlike quantities"

Unlike \Un*like"\, a.

1. Not like; dissimilar; diverse; having no resemblance; as, the cases are unlike.

2. Not likely; improbable; unlikely. [Obsoles.]

   Unlike quantities (Math.), quantities expressed by letters which are different or of different powers, as a, b, c, a^ 2, a^ 3, x^ n, and the like.

   Unlike signs (Math.), the signs plus (+) and minus (-).