Principal or principle?

Principal as an adjective means 'most important':

• The principal reason for the failure to take action was poor communication between government departments. (the most important reason)

We can use principal as a noun to mean the head of a school or college (especially in American English):

• The college principal made a speech congratulating all the students who were graduating in that year.

Principle is a noun. It means 'a rule or theory which explains how something is or works' or 'a moral rule or guideline':

• The scientific principles behind even the most complicated computer are relatively simple.
• He seems to have no principles at all, and is only interested in money. (He has no moral rules or guidelines.)