A person who gives advice on Investments. Those who work for closed-end companies, are prohibited from disseminating advice known to be deceitful or fraudulent, and from acting as a principal on their own accounts by buying and selling securities between themselves and a client without prior written consent. 
Means any person who, for compensation, engages in the business of advising others, either directly or through publications or writings, as to the value of securities or as to the advisability of investing in, purchasing, or selling securities, or who, for compensation and as part of a regular business, issues or promulgates analyses or reports concerning securities. The spellings investment advisor and Investment adviser are both correct. 
From advice c.1300 from avis "opinion, it seems to me, in my view.