DICOM tag
A DICOM tag is the pair of numbers — a group number and an element number — that identifies one specific data element inside a DICOM file, such as (0010,0010) for Patient's Name.
What DICOM tag means in practice
Every attribute in a DICOM object is addressed by its tag rather than by a text label, which is what lets software written independently agree on what a value means. The registry of tags is published as part of the standard: Patient's Name is (0010,0010) with value representation PN, and Study Date is (0008,0020) with value representation DA. Group 0010 holds patient information, 0008 general study information, and 7FE0 the pixel data.
Source
This definition follows NEMA PS3.6 §6 — Registry of DICOM Data Elements, published by NEMA as part of the DICOM standard.
Related terms
Pixel Data
Pixel Data is the DICOM data element, tag (7FE0,0010), that holds the actual image content of an instance, encoded according to the file's transfer syntax.
.dcm file (DICOM Part 10 file format)
A DICOM file stores one image or object using the Part 10 format: a 128-byte preamble, the four-byte prefix "DICM", and then the data elements — and the .dcm extension is a common convention, not a requirement of the standard.
De-identification (DICOM anonymisation)
De-identification is the removal or replacement of patient-identifying data elements in a DICOM object so that the images can no longer be linked to an individual through that metadata.
DICOM
DICOM is the international standard that defines how medical images and their associated patient and acquisition data are formatted, stored and exchanged between imaging devices.
Related on this site: Inspecting DICOM tags in SoftDicom.