an asymmetric (e.g. public-key) cryptosystem, based on modular exponentiation with big exponents and modulus. RSA can be used both for signature and encryption.
RSA is named after its inventors Ronald L. Rivest, Adi Shamir and Leonard Adleman.
The security of RSA depends on the hardness to factor big numbers (e.g. the public key) into its prime factors (to derive the private key).