The term 'once' is used to refer to a single occurrence in the past. The crossword solutions for this term include BYGONE (6 letters), WHILOM (6 letters), QUONDAM (7 letters), ERSTWHILE (9 letters), HERETOFORE (10 letters), PREVIOUSLY (10 letters), EARLIER (7 letters), AGO (3 letters), BEFORE (6 letters), ALREADY (7 letters), LATE (4 letters), FORMER (6 letters), FORMERLY (8 letters), NEE (3 letters), WHEN (4 letters), ERST (4 letters), ERSE (4 letters), ALLAT (5 letters), ASSOONAS (8 letters), ANDFORALL (9 letters), AWHILEAGO (9 letters), UPONATIME (9 letters), INABLUEMOON (11 letters), and ISNOTENOUGH (11 letters). All of these words can be used to describe a single occurrence in the past, with some being more specific than others. For example, 'BYGONE' and 'WHILOM' are more general terms, while 'PREVIOUSLY' and 'AGO' are more specific.