n/a or N/A is a common abbreviation in tables and lists for not applicable, not available or no answer. It is used to indicate when information in a certain table cell is not provided, either because it does not apply to a particular case in question or because the answer is not available.
NA is an initialism which carries the same latter meaning. A slash is not used in an initialism or acronym.
N/A may also refer to Neurotics Anonymous, predecessor of Emotions Anonymous, which uses N/A to distinguish itself from Narcotics Anonymous (NA)