n/a, which uses lowercase letters, is a common abbreviation for not applicable[1] or not available, used to indicate when information in a certain field on a table is not provided, either because it does not apply to a particular case in question or because it is not available. A slash is used to indicate the abbreviation of lowercase words.
NA is an initialism which carries the same latter meaning. A slash is not used in an initialism or acronym. However, the derivative form N/A is in common use.
N/A may also refer to Neurotics Anonymous, predecessor of Emotions Anonymous, which uses N/A to distinguish itself from Narcotics Anonymous (NA)[2]