A darren is a unique maximum-scoring word in a letters round, if there are no other available words of equal length. The term is commonly found in its adjectival form, darrenic, with a darrenic round being one that has only a single available word of maximal length.
For example, in the first-ever episode of Countdown and checking against the dictionary in use at the time, the first round, TNEMARHIB, had no darren, as it had three maxes of equal length: AMBIENT, MINARET and RAIMENT. However, the second round, OCEETSPUG, was darrenic and had TOUPEES as its darren, since there was no other word of equal or greater length available.
The term probably originated with Soo Reams during online play.
Most, but not all, nine-letter words are darrens.
Variants include the rare double darren, triple darren etc., representing a word two, three etc. letters longer than anything else available. Some curiosities include the unlikely but valid round AADFFIITV, which has the quadruple darren AFFIDAVIT, EEEIQTTTU with the quadruple darren ETIQUETTE, and DFGIIIMNW with the quadruple darren MIDWIFING. The word MWAHAHAHA, introduced in the Oxford Dictionaries Online, is also a quadruple darren. The only quintuple darren is the selection AACHHHIUU which gives CHIHUAHUA.