Engineering Sample
A chip revision from within the product development stages, a prototype or beta version. It is a working sample of the planned final product with all its characteristics and properties. Engineering samples are used for development and evaluation of all kinds of issues related to the chip, like electrical specifications and heat dissipation. They are also sent to mainboard- and component manufacturers to provide ways for them to develop their product around an upcoming production version.
Engineering sample chips have markings like 'ES' or 'Sample'. Intel versions also are marked with a Q-Spec number (like Q525 for a Celeron ES) in comparison to the S-Spec product code number of the production version (SL2TR for the same Celeron type). There are also markings like 'not for resale' or 'Intel confidential'.