An endo ice (cold) test fails to identify a tooth that is actually vital (the tooth doesn't respond, but it really is vital). What is this result called?

The tooth is truly vital (the real/true condition is 'positive' for vitality), but the test gave a negative (no-response) result — a test result that incorrectly says 'negative' for a condition that is actually present (vital) is, by definition, a false negative.