The average time between an incident starting and the team being aware of it, the first half of the incident response loop.
MTTD, Mean Time to Detect, is the average elapsed time between an incident starting (a service starts misbehaving in a way users feel) and a responder being aware of it. MTTD is bounded below by your monitoring (you cannot detect what you don't measure) and above by your alert routing (the alert may exist but not reach the right person). Improving MTTD is usually cheaper than improving MTTR, because better signals are easier to install than better remediation tooling.
Industry average MTTD is 4+ hours. That means most teams find out from a customer or a CEO before they find out from their own monitoring. Cutting MTTD from hours to minutes is the single highest-ROI investment most SRE teams can make, because it directly compresses the user-visible duration of every future incident.
See the part of the platform that handles mttd (mean time to detect) in production.