's definition is - is. How to use 's in a sentence. used to form the possessive of singular nouns boy's, of plural nouns not ending in s children's, of some pronouns anyone's, and of word groups functioning as nouns the man in the corner's hat or pronouns someone else's S 1 1. The symbol for sulfur. 2. The symbol for entropy. S 2 abbr. 1. Football safety 2. Bible Samuel 3. satisfactory 4. Saturday 5. Sports shot 6. siemens 7. small 8. soprano 9. a. south b. southern 10. Baseball strike 11. Sports striker 12. Sunday s 1 or S (ĕs) n. pl. s's or S's also ss or Ss 1. The 19th letter of the modern English alphabet. 2. Any S is a statistical programming language developed primarily by John Chambers and (in earlier versions) Rick Becker and Allan Wilks of Bell Laboratories.The aim of the language, as expressed by John Chambers, is "to turn ideas into software, quickly and faithfully". The modern implementation of S is R, a part of the GNU free software project. S-PLUS, a commercial product, was formerly sold by