误诊 means 'to misdiagnose' or 'a misdiagnosis' — a doctor's incorrect diagnosis of a patient's condition. Some dictionaries also list 'delay diagnosis', but in actual usage 误诊 refers almost exclusively to a wrong diagnosis; for a delayed diagnosis you would normally use 延误诊断 or 漏诊. The word can function as either a noun or a verb, and the context will make the part of speech clear.