apos (always lower case) is a key copyediting term[1] for apostrophe which is a mark of punctuation used to indicate possessive case or omission of a letter from a word. In XML it is one of only five predefined character mnemonics.
This leads to lots of confusion when users attempt to use the ' designation in a file. While it is defined in XML and thus in XHTML most browsers do not recognize it.
The code apos is translated and displayed as a single quotation mark ( ' ) in the all character sets but this can cause some confusion in translating punctuation into languages that have different rules and characters for quotations. Thus there is a need to distinguish this character from quotations in typography. Automated conversion tools are often incapable of distinguishing an apostrophe from single quotations, particularly when it is used as a substitute for the first or last letter of a word.