In BRF parlance, this is the nickname for the big Sukhoi Su-30 MKI fighter jet.
The original Rambha is a mythological dancer and singer, who dances for the Gods in Hindu and Buddhist traditions. This name was later adopted as a movie-name by a South Indian actress known for her large (ahem) assets and her B-movie vamp roles. When the SU-30 pictures were first posted onto the forum, one senior member remarked that the exhaust vents in the rear of the jet (which are pretty large, as the SU-30 MKI is a big airplane) reminded him of the movie star's rather fleshy gams. He started referring to the airplane as "Rambha" and the nickname stuck.
The nickname is an unofficial one and not seen on other forums or news media. However, word has reached that some readers of BRF who also serve in the Indian Air Force, also call the airplane by its BRF nickname!