As curious user said in a other answer *probabilistic* means that the encryption of the same plaintext under the same key gives as output a different ciphertext.

This is a more general property and it is known as a basic security property. It was firstly defined and introduced by Goldwasser and Micali in their seminal paper "[Probabilistic Encryption][1]" on 1984. This property is usually achieved introducing a random token into the encryption algorithm.


  [1]: http://theory.lcs.mit.edu/~cis/pubs/shafi/1984-jcss.pdf