Intellectual activity that serves no practical purpose. Excessively theoretical, and therefore a distraction from more practicable matters.
Dr. Richard Dawkins, noted biological theorist, in response to a student's philosophical question about skeptical attitudes towards science and reality, cited the following anecdote:
James Boswell asked, "... nothing really exists unless there's somebody there to see it. How do you refute that?" Samuel Johnson replied, "I refute it thus!", and kicked a stone out of his way.
Dr. Dawkins, commenting on Boswell's question, and critiquing the attitude: "If you want to mess about with that sort of mental masturbation, thats fine, but, ... but the science of reality is what gets you through the day, ... and makes things work!"