Farther India, or Ultraindia, is an old term, now rarely used, for Southeast Asia, seen in colonial days from Europe as the part of the Far East beyond the Indian subcontinent, but south of China.
It refers to Indochina (Vietnam, Cambodia, Laos), Thailand (former Siam), Burma/Myanmar, and the Malay states (Indonesia, Malaysia, Brunei and Singapore), but usually not to the Philippines or East Timor, neighbouring predominantly Malay states which belong to the wider East Indies (which includes all the above as well as the Indian subcontinent).
Other use
Farther India is also a title of a book written by Sir Hugh Clifford.
References
Farther India Wikipedia(Text) CC BY-SA