RSA (Rivest–Shamir–Adleman) 1) is a public-key cryptosystems implementation (one of the first).
RSA is then a asymmetric cipher (ie public key scheme).
RSA uses exponentiation modulo a product of two very large primes, to encrypt and decrypt, performing both:
Its security is connected to the extreme difficulty of factoring large integers, a problem for which there is no known efficient general technique.
The RSA algorithm is named for its inventors: Rivest, Shamir, and Adleman. 2)