Locant

Part of speech: noun

Definitions

1. A number or letter that specifies the position of an atom or substituent within the name of a chemical compound
2. A numerical or alphabetic marker showing where a particular group is attached on a molecule's structure
3. A position indicator, typically a number or letter, used in naming organic compounds to identify where a substituent sits

Etymology: "Locant" is a chemistry term for the number or letter that tells you where a substituent sits on a molecule's carbon chain or ring. In the name 2-chlorobutane, the "2" is the locant. The word is built from Latin "locus," meaning "place," with the ending "-ant," which forms nouns for something that performs an action or fills a role. A locant, in other words, is the thing that marks a place. The link to "locus" is clear enough that the word reads almost like a transparent label, and it fits the naming system used by chemists, where each position on a molecule has to be identified unambiguously. The Latin root also appears in English words such as "locale" and "location," which share the same core idea of a specific spot.