A lisp is a speech impediment. Stereotypically, people with a lisp are unable to pronounce sibilants (like the sound [s]), and replace them with interdentals (like the sound [θ]), though there are actually several kinds of lisp. In one of these, the sounds become lateralized; the symbols for this in the Extended International Phonetic Alphabet for speech disorders are [ʪ] and [ʫ].
Notable people that had or have lisps include Thomas Jefferson (who preferred writing to public speaking partly because of this), the rapper Anybody Killa, Mike Tyson, and Nat King Cole (in his early career). John Adams also had a lisp in his later years, but this was because he refused to wear dentures. The biblical figure Moses is described as having "slow speech and a slow tongue" (Exodus 4:10), which is traditionally interpreted as a lisp, though others have believed it was a stutter or merely hesitancy. Winston Churchill had a slight lisp, which is often incorrectly said to have been a stutter.