You may get surprised when you see Mp5 player on sell if you are a CS(counter strike by valve)fan. When you google "mp5",You will get most result as a machine gun you imagined before. However, you may get familiar with mp3 player(also DAP, digital audio player),mp4 player(also PMP, portable media player support MPEG-4 or other video formats).now you may guess those simple 3+1=4 and 4+1=5.That is right, Then what is Mp5 player, unluckily, the is no such authoritative definition on the web, However, the word "Mp5" have been invented in 2006.But different factories have difference opinions on this "future" item. Let see what it is:

Features :Support RM and RMVB format(usually played by realplay) which composed of large part of web videos. It is mainly used for watching web videos transported from your PC to PMP. With the growing digital broadcasting network, Some Mp5 players add new features as watching mobile TV as TV cell phones. Plus, as the next generation of Mp4 player, Mp5 support those picture viewing, text reading, audio playing, video playing FM radio functions additional camera functions and transmitting interface like Bluetooth and infrared(attention, not all function will be in one,).

Technology:
Mp3: An audio compression technology from the mpeg specification that enables cd quality sounds to be compressed which reduces file size while maintaining it's high fidelity quality. A widely used name for MPEG-1 Audio Layer-3, the technology that allows compressed-format audio files to be sent over the Internet.
Mp4: MPEG-4 , is a multimedia container format standard specified as a part of MPEG-4. It is most-commonly used to store digital audio and digital video streams, especially those defined by MPEG, but also can be used to store other data such as subtitles and still images. However, most Mp4 players don’t really play Mp4 files, and people believe the Mp4 player is Mp3 player with video playing function being watched on large display screen and large storage.
Mp5: An acronym for MPEG layer 5 ,A new compress technology invented by Chinese factories producing DAPs in order to support more video format, save more storage space and protect copyright.

It gets really hard to make a entity definition, a company even invented "Mp5 cell phone", gun , phone, or both? we are becoming more and confused.