advisor

An advisor is somebody whose job is giving advice. A financial advisor, a special advisor, my advisor.

Both spellings are correct, adviser and advisor, with adviser older and commoner in British newspapers and advisor commoner in job titles and in America — the -or looks more official, which is much of why it spread. In British politics a special adviser is a political appointee, known as a spad. Note that advisory as an adjective and a noun both exist: an advisory role, a weather advisory.