Quote from American Heritage Dictionary:

"The act of asserting or declaring. A declaration stated positively with no support or attempt at proof. "

Description:

ASSERTION stands triumphant in his reversed pyramid, equalizing his spirit with matter. The male youth recognizes his strength and glories in the power of it; his energy is boundless. Since there is no war within, there is nothing to fight. This youth has discovered that cosmic energy needs both positive and negative charges, and he drives his duality with certainty. The two lions of the polarities pull his chariot. ASSERTION seeks to understand himself by harnessing his inner energy. He is the essence of humanity to become.

Interpreted use:

Your polarities are in harmony; stay with the process. Good luck!

Reversed (card received upside down):

You have yet to learn about service to others, rather than to yourself. If you do not serve, your personal aggression will drag your Serplions to a standstill.