What is Blu-ray?

Legal Definition
Blu-ray or Blu-ray Disc (BD) is a digital optical disc data storage format. It was designed to supersede the DVD format, in that it is capable of storing high-definition (720p and 1080p) and ultra high-definition video resolution (2160p). The plastic disc is 120 mm in diameter and 1.2 mm thick, the same size as DVDs and CDs. Conventional (pre-BD-XL) Blu-ray Disc discs contain 25 GB per layer, with dual layer discs (50 GB) being the industry standard for feature-length video discs. Triple-layer discs (100 GB) and quadruple layers (128 GB) are available for BD-XL re-writer drives. The name "Blu-ray" refers to the blue laser (specifically, a violet laser) used to read the disc, which allows information to be stored at a greater density than is possible with the longer-wavelength red laser used for DVDs. The main application of Blu-ray is as a medium for video material such as feature films and physical distribution of video games for the PlayStation 3, PlayStation 4, and Xbox One. Besides the hardware specifications, Blu-ray is associated with a set of multimedia formats.

High-definition video may be stored on Blu-ray discs with up to 2160p resolution (3840×2160 pixels), at up to 60 frames per second. DVD discs had been limited to a maximum resolution of 480i (NTSC, 720×480 pixels) or 576i (PAL, 720×576 pixels). The format was developed by the Blu-ray Disc Association, a group representing makers of consumer electronics, computer hardware, and motion pictures. Sony unveiled the first Blu-ray disc prototypes in October 2000, and the first prototype player was released in April 2003 in Japan. Afterwards, it continued to be developed until its official release in June 2006. During the high definition optical disc format war, Blu-ray Disc competed with the HD DVD format. Toshiba, the main company that supported HD DVD, conceded in February 2008, releasing its own Blu-ray Disc player in late 2009. According to Media Research, high-definition software sales in the US were slower in the first two years than DVD software sales. Blu-ray faces competition from video on demand and the continued sale of DVDs. As of January 2016, 44% of U.S. broadband households had a Blu-ray player.
-- Wikipedia
Legal Definition
A storage device that has high quality video. Sony invented this format of DVD. A special player is needed.