Doctor of Philosophy
A PhD (Latin Philosophiae doctor) is a degree attained by grad students in the arts and sciences after years and years of thankless toil - usually followed by years and years of thankless toil in the attempt to hold down an academic career.
At Columbia, they are awarded by the Graduate School of Arts and Sciences. That is, even when one's advisor is in, for example, SEAS, a PhD can only be awarded by GSAS. SEAS, for example, is able to award Doctor of Engineering Science degrees, but not PhDs.