a person who gives some advice to other that need some guidance; a counsellor;
a knowledgeable person or a company that is often approached by businesses or government offices or even by an individual person for some guidance and/or the know-how of certain things in certain fields; a consultant;
(mostly in Britain) an educated and trained officer in education department who gives some guidance to the head-teachers and teachers of a school on the new and suitable methods of teaching;
an educated and trained officer or a senior teacher whose job is to advise students on academic issues and future job opportunities,
There are several types of advisers: trading adviser, advises on how to invest money on which business; military advisers, advises solders on how to fight a battle and how to use modern weapons; technical adviser, advises on the latest technological developments and the suitable technology to be used; financial adviser, advises on how to manage the money a person earns or saves or invests; student adviser, advises the students on their education and job opportunities; and health adviser, advises on how to keep ourselves healthy and which hospital to go to when in need.
A ‘senior adviser’ in some countries is an official who is appointed by the government to guide and supervise certain government departments.