Please see below:

A). http://dictionary.reference.com/browse/adjunct
6 results for: adjunct
2. a person associated with lesser status, rank, authority, etc., in some duty or service; assistant.
3. a person working at an institution, as a college or university, without having full or permanent status: My lawyer works two nights a week as an adjunct, teaching business law at the college.

B). http://en.wikipedia.org/wiki/Professor

Visiting professor: someone visiting another college or university to teach for a limited time; this may be someone who is a professor elsewhere or a distinguished scholar or practitioner who is not. The term may also refer simply to terminal (usually 1 to 3 years) teaching appointments and/or post-doctorate research appointments (which are much like research internships). See also: Sessional instructor.

Adjunct professor: someone who does not have a permanent position at the academic institution; this may be someone with a job outside the academic institution teaching courses in a specialized field; or it may refer to persons hired to teach courses on contractual basis (frequently renewable contracts); it is generally a part-time position with a teaching load below the minimum required to earn benefits (health care, life insurance, etc.), although the number of courses taught can vary from a single course to a full-time load (or even an overload).

An adjunct is generally not required to participate in the administrative responsibilities at the institution often expected of other full-time professors, nor do they generally have research responsibilities. The pay for these positions is usually nominal, even though adjuncts typically hold a Ph.D., but most adjuncts also hold concurrent positions at several institutions or in industry.

Adjuncts provide flexibility to the faculty, acting as additional teaching resources to be called up as necessary, however, their teaching load is variable: classes can be transferred from adjuncts to full-time professors, classes with low enrollment can be summarily canceled and the teaching schedule from one semester to the next can be unpredictable (furthermore, if the university makes a good faith offer to an adjunct professor of teaching during the following semester (dependent on enrollment), the adjunct generally cannot file for unemployment during the break). In some cases, an adjunct may hold one of the standard ranks in another department, and be recognized with adjunct rank for making significant contributions to the department in question.

C). 新英汉词典(上海译文出版社，1973), adjunct professor(美国某些高等学校的副教授)