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.
What Pixel Data means in practice
Everything else in a DICOM file describes this element: the rows and columns say how to lay it out, the bits allocated say how wide each sample is, and the photometric interpretation says how to colour it. Because its encoding follows the transfer syntax, the same anatomical image can be stored uncompressed, JPEG Lossless or JPEG 2000 and remain the same Pixel Data element.
Source
This definition follows NEMA PS3.5 §8 — Encoding of Pixel Data, published by NEMA as part of the DICOM standard.
Related terms
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.
Lossless compression (DICOM)
Lossless compression reduces the size of DICOM Pixel Data while allowing the exact original pixel values to be reconstructed, unlike lossy compression, which permanently discards image information to achieve a smaller file.
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.