In the context of the RSA cryptosystem, the "R" in RSA stands for Ron Rivest, one of the three inventors of the algorithm along with Adi Shamir and Leonard Adleman. The RSA cryptosystem is a public-key cryptosystem that is widely used for secure data transmission. It involves a public key and a private key, where the public key can be known by everyone and is used for encrypting messages, while the private key is used for decrypting messages. The public key is represented by the integers n and e, and the private key by the integer d).