Introspection

(skt.: samprajanya, tib.: she shin) Sometimes also translated as vigilance, watchfulness, awareness, alertness, attentivness... It is a mental factor, which functions like a mental alarm which warns when you lose your object of mindfulness or concentration. 

Vasubandhus Commentary on the Ornament for the Mahayana Sutra states: Mindfulness and introspection bring about close mental focus because the former prevents your attention from wandering from the object of meditation and the latter clearly recognizes that your attention is wandering.