A darren is the best possible word in a letters round, without equal. From the selection ALEPOCQTS, POLECATS would be the darren, as there is no other word of equal or greater length. However, MATAAENIH, which has three maxes of equal length (ANATHEMA, ANTHEMIA and HAEMATIN), has no darren.
Like most other things in life, the term probably originated with Soo Reams during online play. It is also commonly found in its adjectival form, darrenic, with a darrenic round simply referring to a round that has a darrenic max.
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 EEEIUQTTT, which has the quadruple darren ETIQUETTE, and GMWIIIDFN with the quintuple darren MIDWIFING. The word MWAHAHAHA, introduced in the Oxford Dictionary Online, is also a quintuple darren.