一旦 is pronounced yídàn and means “once; in case”. It is one of the words in the HSK 5 vocabulary, a level of the Hanyu Shuiping Kaoshi, China’s standardised Chinese proficiency test. Recognising a word on a page and recalling it when you need it are different skills, and only the second survives a conversation — which is why LexiTrail shows you 一旦 again just before you would have forgotten it, rather than on a fixed schedule.