The word with two letters is Mrityu (death,) and the word with three letters is Shashvata Brahman, the eternal spirit. The consciousness of ego, or the state of being addicted to worldly objects is Death, and the absence of that feeling is immortality.
व्यक्षरस्तु भवेन्मृत्युस्त्र्यक्षरं ब्रह्म शाश्वतम्। ममेति च भवेन्मृत्युर्न ममेति च शाश्वतम्॥