The habit of treating philosophical or theoretical problems as though they were psychological ones, to be solved by methods such as introspection.
Properly speaking it is only a theory when engaged in deliberately rather than, as more often, unconsciously or through confusion, though the distinction is not sharp.
Psychologism is common in the early empiricists like John Locke (1632-1704) and also (especially in arithmetic) John Stuart Mill (1806-1873), and indeed in some British universities psychology and philosophy were only finally disentangled after World War II (paradoxically, perhaps, because philosophy has since moved towards closer contact with the sciences, and the line between them -and hence what counts as psychologism - has become more blurred).
Psychologism was, however, vigorously criticized by Francis Herbert Bradley (1846-1924), Gottlob Frege (1848-1925), and Edmund Husserl (1859-1938) in his later works. In any case psychologism should not be confused with naturalized epistemology, which explicitly replaces philosophical questions with psychological ones.