LYBNTS (pronounced lie-bunts) and SYBNTS (pronounced sigh-bunts) are not characters from an episode of Dr. Who nor are they arcane baseball terms, but they are acronyms of potential gold for your annual fund.
LYBNTS are people who gave Last Year But Not This. SYBNTS are people who gave Some Year But Not This. At some point, these people cared enough about your mission to support it financially and, with careful cultivation, they may care enough to give again.