N/A correctly n/a , is a common abbreviation for not applicable, 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. Can also mean no answer under certain situations.  Being not appropriate or relevant; not able to be applied; It does not fit. 
It shouldn't be written as "N/A." The slash (/) is also known as: forward slash, stroke, oblique is used with certain abbreviations: n/a not applicable, not available is one of them. The slash indicates that the lowercase letters are an abbreviation. 
1560s applicable from "pliable, capable of being applied is attested by 1660; earlier in this sense was appliable (1550s). Applicability (1650s).