Terminus ante quem and terminus post quem: two expressions to indicate a relative chronology.
Terminus ante quem simply means "moment before which X happened" and terminus post quem means "moment after which X happened". For example, if we know that the Armenian king Artaxias supported a revolt in the Seleucid Empire that broke out in 162 BCE, this is a terminus post quem for Artaxias' death, because it must have happened before he died. Dendrochronological dates are always post quem. In archaeology, if a higher stratum can be dated, this date is a terminus ante quem for lower strata.