Brightline

A brightline is a specific, clearly-understood "line in the sand" used to decide the nature of something. For example, rather than subjectively asserting that a tax cut is or isn't significant, a debater might propose a brightline: a tax cut is significant if it is more than 3%. This simplifies the argument, because the question is now about whether 3% is a good level or not, which is something that can be answered more definitively.

Brightlines are most often used as part of topicality arguments.