Large-scale eruption events in the solar atmosphere can generate global waves, i.e., waves that propagate over distances comparable to the solar radius. In the low solar corona, global waves observed by SOHO EIT, generated by coronal mass ejections or flares, are usually referred to as "EIT waves." The nature of these global waves is the subject of strong debate, and opinions are divided between different interpretations (e.g., fast magneto-hydrodynamic waves, shock waves, nonwave feature, etc.). In the present Letter, we studied TRACE EUY data to show that these global coronal disturbances are indeed waves with a well-defined period. Supposing that the EIT waves transfer all their energy to interacting loops, we also estimate the minimum energy threshold carried by EIT waves.