The ability to use symbolic representation or logic. The opposite of abstraction, according to Carl Jung, is concretism.
Example: A middle schooler is struggling with his algebra homework; he can’t seem to wrap his head around the concept of variables.
Grinnell, R. (2008). Abstract Thinking. Psych Central. Retrieved on April 21, 2015, from http://psychcentral.com/encyclopedia/2008/abstract-thinking/