A multimedia file that contains content sourced from a Blu-ray movie disc.
To make a BDRip, you need to copy the main movie of a blu-ray disc to your hard drive, and then re-encode it to reduce size. BDRips are usually Matroska files, with MPEG 4 AVC video, high quality AAC audio, and possibly subtitles and chapters (this type of file is usually called a "DivxHD" file). BDRips always come in HD, aka 1280x720 resolution or above (MKV files in standard definition are not considered BDRips).
BDRips are the next thing after DVDrips
(which are always standard def).
The problem is that, while DVDRips use a video format superior to the one the source uses (MPEG 4 ASP vs MPEG 2 of the DVD), which means it's possible to dramatically reduce the size of the rip compared to the original, BDRips use the same video format as Bluray (MPEG 4 AVC in both cases). This means it's not possible to reduce size that much, if you want to preserve the HD resolution and video quality.
Anyway, BDRips are several times better in terms of quality than any DVDrip. BDRips are also bigger in size. They usually come in two resolutions, 720p (4,35GB~6GB in size) and 800p (7,9GB~13GB in size, often called "1080p" vids).