LUT is the acronym for Look Up Table.  This is a table with x and y coordinates used to map data into an other domain.

Commonly used LUT's are:

• Modality LUT for mapping acquired data into a modality independent domain, e.g. as sent by a CR device.
• VOI LUT, which does not always exist of a LUT but also can be represented by a WW and LVL (in case of linear LUT)
• Presentation LUT, typically used for printers to adjust the print quality and/or have it comply with the GSDF.
• In many cases the LUT is specified as "IDENTITY" which means that there is a 1:1 relationship between the input and output values.