1610s, agent noun from advise (v.). Meaning "military person sent to help a government or army in a foreign country" is recorded from 1915. Alternative form, Latinate advisor, is perhaps a back-formation from advisory.
1. She refused to promote Colin above the low rank of "legal adviser".
2. If you are in any doubt, consult a financial adviser.
3. He was relieved of his duties as presidential adviser.
4. He was their adviser, confidant and father confessor.
5. a special adviser to the President on education