difficulty in experiencing, expressing, and describing emotional responses.
From the Greek a + lexi + thymia -- literally "without words for emotions." A term coined by psychotherapist Peter Sifneos in 1973. (paraphrased from Wikipedia)
Alexithymia is not a diagnosis, but a construct useful for characterizing patients who seem not to understand the feelings they obviously experience, patients who seem to lack the words to describe these feelings to others.
--When a Patient Has No Story to Tell: Alexithymia (Psychiatric Times)