ध्वजः।
Flag is called ध्वजः (dhvajaḥ) in Sanskrit.
If you specifically wanted to mention national flag then you can call it राष्ट्रध्वजः (rāṣṭradhvajaḥ). राष्ट्र (rāṣṭra) means nation ध्वज (dhvaja) means flag and the word combined means national flag in Sanskrit.
पताका (patākā) is a similar word for flag in Sanskrit.
