MrSID is an acronym for Multi-resolution Seamless Image Database

I use MrSID files a lot in GIS.

MrSID is a powerful wavelet-based image compressor, viewer and file format for massive raster images that enables instantaneous viewing and manipulation of images locally and over networks while maintaining maximum image quality. Features include unprecedented compression ratios while maintaining highest image quality, true multiple resolutions, selective decompression, seamless mosaicking and browsing.