Adviser vs. advisor

In answer to a student's question:

Adviser and advisor are both accepted spellings of the noun meaning one who advises or counsels. There is no difference between them. But adviser, the older version, is listed as the primary spelling in most dictionaries, and it is about five times as common as advisor in current news publications throughout the English-speaking world.

In the U.S. and Canada, advisor is commonly used in official job titles, but adviser is still generally preferred over advisor in North America, and advisor is only marginally more common in American and Canadian English than in other varieties of English.

http://grammarist.com/spelling/adviser-advisor/