Is Indochina a peninsula?why is it called a peninsula?

Geographically speaking, Myanmar, Thailand and Malaysia are part of South East Asia, as are the Phillipines, Brunei, Singapore, Indonesia, the Malay archipelago, East Timor and the three countries usually referred to as Indochina.  Although mainland Southeast Asia is sometimes referred to as the "Indochinese Peninsula" the term Indochina correctly refers only to the areas colonized by the French in the 19th century, which today are the nations of Laos, Cambodia and Vietnam. The term "Indochina" itself is French. The South China Sea lies to the east of Vietnam, and the Gulf of Siam lies south of the coast of Cambodia.  Malaysia is a peninsula stretching southwards from Thailand toward Sumatra, but calling Indochina per se (the three former French colonies) a peninsula would be stretching the definition pretty far.  It would be sort of like calling the southeastern United States a peninsula because the Atlantic Ocean is to the east and the Gulf of Mexico is to the south.