This is called in full Doctor of Philosophy, but is usually referred to as PhD. The word order is that of the original Latin philosophiae doctor
A PhD is awarded on acceptance of a thesis which must be an original contribution to knowledge. Research for this degree usually takes about three years, although the length of time needed varies considerably according to the subject. This degree is generally considered to be of an equivalent standard to the Russian кандидатская степень.
There is another type of doctorate, which is called in full a senior doctorate, to avoid confusion with the PhD. The name of a particular senior doctorate depends on the field of specialization.
e. g. Doctor of Letters- for arts subjects or DLitt- from the Latin doctor litterarum; Doctor of Science- for science subjects or D. Sc.
These degrees are much higher than the PhD and comparable in importance to the Russian doctor's degree. However they differ from the latter in that they do not involve the writing of a thesis. A person wishing to apply for such a degree submits his published works to a board, or committee, who then decide whether these works justify the award of the degree.
(Based on J. Povey and I. Walshe, An English Teacher's Handbook of Educational Terms, Moscow, "vyssaja Skola", 1982)