.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.
What .dcm file (DICOM Part 10 file format) means in practice
The prefix is what actually identifies the file, which is why a DICOM file with no extension at all, or one named IM00001, is still perfectly valid and still opens. The standard mentions ".dcm" once, as an optional alternate file name following RFC 3240; files on a patient CD frequently have no extension because the DICOMDIR index names them instead. A viewer should therefore read the prefix rather than trust the file name.
Source
This definition follows NEMA PS3.10 §7.1 (preamble and DICM prefix), §7.3 (file naming), published by NEMA as part of the DICOM standard.
Related terms
DICOMDIR
DICOMDIR is the index file at the root of a DICOM media set, such as a patient CD or USB export, that lists the patient, study, series and instance records the media contains so a viewer can browse it without opening every file.
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.
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.
Transfer Syntax
A transfer syntax is the set of encoding rules — byte ordering, explicit or implicit value representation, and any compression — that a DICOM object is written in, and it is negotiated between two devices before they exchange data.
Related on this site: Open a .dcm file with SoftDicom.