A professor is a university lecturer of the highest rank.

The title is not an academic qualification but a rank in the university. The highest academic qualification rather is PhD (Doctor of Philosophy).

Professor Morten Meldal, Nobel Prize winner in Chemistry

Some tertiary institutions other than universities also give the rank of professor to their lecturers but in most cases it is universities that do it.

Before one becomes a professor they might have passed through other levels.

Professor Wole Soyinka, Nobel Prize Winner in Literature

The first level of qualification is to have a doctorate degree in the area of specialization.

There are four initial levels:
Lecturer Grade 2 (also known as Lecturer II),
Lecturer Grade 1 (also known as Lecturer I),
Senior Lecturer
and Associate Professor.

After the rank of Associate Professor one then moves to the rank of Full Professor

Hence, before being considered for promotion or appointment to the rank of full Professor, the academic/scholar would have put in a minimum of twelve years of university teaching and research.

The third and about the most important filter is the assessment of scholarship.

This involves evaluation of the published works of the full Professorial candidate by seasoned (senior) full Professors in the field of the candidate.