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Adjunct professor (adjunct lecturer, adjunct instructor, adjunct faculty or contingent academic labor collectively) is a type of academic appointment in higher education.
In the United States, an adjunct is a non-tenure-track faculty, though it can also be a scholar or teacher whose primary employer is not the school or department with which they have adjunct status.  In most cases adjunct professors need a master's degree, but in some cases only require a bachelor's degree and relevant experience.
In Canada, adjunct professors are often nominated in recognition of active involvement with the appointing institution, while they are employed by government, industry, a profession or another institution. The position course lecturer, rather than adjunct, is used if the appointment is strictly to teach one or more courses, contrasting with US usage of the title for all instructors.
In Portugal the designation professor adjunto implies a stable full-time employment in a polytechnic university. Notably, in countries such as Argentina and Brazil, a similar designation, profesor adjunto, also implies a stable employment. The same term used in Argentina and Brazil refers to a non-tenured position in parts of Spain.
In Finland, the Docents' Union of Finland and the Finnish Ministry of Education recommend the term adjunct professor in English as a translation of the title of docent. A docentship should be regarded as an educational title not connected with the employment pyramid as such, rather an assurance of the level of expertise, to enable the person to advance further in their academic career. The rank of a docent entitles scientists to be principal investigators, lead research groups and act as the supervisors of doctoral students.
In Bangladesh, private universities follow the title adjunct professor or adjunct associate professor to imply non-tenure faculty members.
In Pakistan, adjunct (assistant/associate) professors are also considered as non-regular faculty members and usually posts are given to Pakistani overseas scientists under a faculty development program.
In Australia, the term adjunct is reserved for academics and researchers from outside the university who have a close association with the university, e.g. through supervision of PhD students, recognised by an honorary title reflective of their rank and standing (adjunct lecturer, senior lecturer, associate professor or professor). 
In Thailand, adjunct (assistant/associate) professors are considered "non-regular officers".