The symbol kyr was formerly common in some English-language works, especially in geology and astronomy, for the unit of one thousand years or millennium. The "k" is the unit prefix for kilo- or thousand with the suffix "yr" simply an abbreviation for "year".
Occasionally, the "k" is shown in upper case, as in "100 Kyr"; this is an incorrect usage. "kyr" itself is often considered incorrect, with some preferring to use "ky"
Modern, ISO 31-1 recommended usage is ka for kiloannum, which avoids the implicit English bias of "year" by using a Latin root.