V. S. Apte, The Practical Sanskrit–English Dictionary. Look up any word of the Gita in Devanagari, IAST or Harvard-Kyoto, or browse the headwords letter by letter.
yasyāṁ jāgrati bhūtāni sā niśā paśyato muneḥ. — That (higher being) which is to all creatures a night, is to the self-mastering sage his waking (his luminous day of true being, knowledge and power)… 2·69
साsānominative singular of the feminine form of the pronoun tad (that).