A number that identifies a specific DICOM Data Element and defines the data element name, content and data type.
The tag is a long number (4 bytes) comprised of two short numbers (2 bytes each), group and element. Related tags usually share the same group, for example patient data elements belong to group 10.
Tags usually appear in documentaion as a pair of two 4 hexadecimal digits in the following form: (gggg,eeee) where gggg is the group and eeee is the element.
A full list of standard DICOM tag can be found in chapter 6 of the DICOM standard (Data Dictionary).
DICOM defines a mechanism for extending the dictionary with private tags that can be used for proprietary data elements.