In context of public key encryption it is not meaningful to consider known-plaintext attacks in the way this is done in context of symmetric encryption schemes (where an adversary gets hold of ciphertexts and knows the corresponding plaintexts and wants to recover the secret key). In public key encryption everyone (as the public key is public) can produce ciphertexts for arbitrary messages of his choice.
However, in the formal treatment of public key encryption, there is a model called indistinguishability under an eavesdropping attack, which essentially says that an adversary is allowed to choose two messages, then gets the ciphertext for one of these messages and has to guess which one has been encrypted.
As, however, in public key encryption, the adversary can built its own encryption oracle by means of knowledge of the public key, this aforementioned notion is equivalent to the well known notion of indistinguishability under chosen plaintext attacks (IND-CPA). This attack game is essentially identical to the aforementioned, besides that the adversary has access to an encryption oracle throughout the security game (which is more meaningful).
In the case of textbook RSA, an adversary can use the strategy that is called "forward search attack" in the HAC (this is an attack strategy that can be used but no security notion) to win the IND-CPA game. More precisely, when receiving the challenge ciphertext (one of the two submitted messages), then the adversary simply calls the encryption oracle for one of the two ciphertexts. If the response ciphertext of the oracle is identical to the challenge ciphertext, the adversary knows that the message submitted to the oracle is in the challenge ciphertext, and the other one otherwise. This attack strategy is feasible for every deterministic public key encryption scheme (such as textbook RSA), where multiple encryptions of the same message with respect to the same public key always yield the same ciphertext.
In context of public key encryption schemes the weakest notion of security considered today is IND-CPA (which is equivalent to the above mentioned eavesdropping) and the common stronger notions are indistinguishability under chosen ciphertext attacks (IND-CCA1) and a indistinguishability under adaptively chosen ciphertext attacks (IND-CCA2). In the latter two security games the adversary has additionally access to a decryption oracle during the security game (only before seeing the challenge in IND-CCA1 and also after seeing the challenge in IND-CCA2).