An adviser or advisor is normally a person with more and deeper knowledge in a specific area and usually also includes persons with cross-functional and multidisciplinary expertise. An adviser’s role is that of a mentor or guide and differs categorically from that of a task-specific consultant. An adviser is typically part of the leadership, whereas consultants fulfill functional roles.
The spellings adviser and advisor have both been in use since the 16th century. Adviser has always been the more usual spelling, though advisor has gained frequency in recent years and is a common alternative, especially in North America.