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.
What DICOMDIR means in practice
It is a DICOM object in its own right, holding a directory of records that point at the image files elsewhere on the media. Without it a viewer has to read the header of every file to work out what is on the disc, which is slow on a large study; with it the viewer can show the study list immediately. The file name DICOMDIR is reserved by the standard for exactly this purpose.
Source
This definition follows NEMA PS3.10 §8.6 — Basic Directory (DICOMDIR) File ID, published by NEMA as part of the DICOM standard.
Related terms
.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.
Patient, Study, Series and Instance
DICOM organises imaging data in a four-level hierarchy: a Patient has Studies (one imaging visit each), a Study has Series (one acquisition run each), and a Series has Instances (the individual images or objects).
Related on this site: Reading a patient CD or USB study.