Mark up acronyms with acronym and use title to indicate the expansion.  At a minimum acronyms must be marked where they first occur in the delivery unit and after every entry point into the delivery unit.  An entry point into the delivery unit is each identifed anchor which can be navigated to via a URI. 

A best practice is to mark all occcurences of acronyms within the delivery unit. In addition, acronyms are abbreviations and the <abbr> element can be used instead of acronym. Developers intending to transition to XHTML should note that  XHTML 2.0 removes the acronym element in favor of abbr and indicates that all occurrences should be marked. 

The English language definition of acronyms varies. WCAG has accepted the definition that defines acronyms as words formed from the first or first few letters of existing words. These acronyms are pronounced using syllables rather than speaking each individual letter separately.  Thus, NASA,  for National Aeronautices and Space Agency, is an ancronym but MDC, Metropolitan District Commission, is just an abbreviation.