A Global Trade Item Number (GTIN). GTINs identify trade items, including products and services, using numeric identification codes.
The GS1 digital link specifications express GTINs as URLs (URIs, IRIs, etc.). Details including regular expression examples can be found in, Section 6 of the GS1 URI Syntax specification; see also schema.org tracking issue for schema.org-specific discussion. A correct gtin value should be a valid GTIN, which means that it should be an all-numeric string of either 8, 12, 13 or 14 digits, or a "GS1 Digital Link" URL based on such a string. The numeric component should also have a valid GS1 check digit and meet the other rules for valid GTINs. See also GS1's GTIN Summary and Wikipedia for more details. Left-padding of the gtin values is not required or encouraged. The gtin property generalizes the earlier gtin8, gtin12, gtin13, and gtin14 properties.
Note also that this is a definition for how to include GTINs in Schema.org data, and not a definition of GTINs in general - see the GS1 documentation for authoritative details.