Euclid-class SCPs are anomalies that require more resources to contain completely or where containment isn't always reliable. Usually this is because the SCP is insufficiently understood or inherently unpredictable.

Euclid is the Object Class with the greatest scope, and it's usually a safe bet that an SCP will be this class if it doesn't easily fall into any of the other standard Object Classes.

As a note, any SCP that's autonomous, sentient and/or sapient is generally classified as Euclid, due to the inherent unpredictability of an object that can act or think on its own.