The definition of Itihasa is इतिह आस्ते अस्मिन् - it contains advice for leading a meaningful life. They are based on real incidents of the past. The definition of Purana is पुरातनवृत्तान्तकथनम् - Puranas are narrations of past incidents. Itihasas are centered around a particular theme; Ramayana around Rama's life and Mahabharata around the Kuru Vamsha. A book can be called Purana only if it contains the following five: 1. Sarga - description of primary creation. 2. Pratisarga - description of secondary creation. 3. Vamsha - description of important Vamshas such as Surya Vamsha and Chandra Vamsha. 4. Manvantara - description of the 14 Manvantaras, Manus, Devas, Rishis, and avataras. 5. Vamshanucharita - description of the important persons in the Vamshas. पुराणं पञ्चलक्षणम् - Puranas should have these five characteristics.