Sigmatism is a linguistic term referring to the pronunciation of the letter 's' as a 'z' sound. It is also known as sibilant assimilation, and is commonly seen in languages such as English, Spanish, and French. The crossword solution for this term is 'LISP', which stands for 'LISt Processing'. This is a computer programming language that was developed in the late 1950s and is still used today.