Definition - What does x264 mean?
x264 is an open-source library developed by VideoLAN to allow video streams to be encoded into the MPEG-4 AVC/H.264 format. Released under the terms of the GNU General Public license, x264 can also be availed under the commercial license from x264LLC and CoreCodec.
Techopedia explains x264
Compared to other H.264 encoders, x264 provides a comparatively large number of features. It comes with an API as well as a command line interface. The command line interface for x264 is used by many graphical user interfaces like MeGUI and Staxrip, whereas the API is used by interfaces such as FFmpeg and HandBrake. For enhancing the subjective video quality of the encoded video, x264 comes with psychovisual enhancements such as the pyschovisual rate-distortion optimization and adaptive quantization available in two modes. One of the salient features of x264 is its ability to use Periodic Intra Refresh, enabling every single frame to be capped at the same size, instead of using keyframes. This further helps each slice to be transmitted quickly in a single TCP or UBP packet and also allows faster decoding on arrival.
x264 is capable of achieving dramatic performance with encoding four or more 1080p streams in real time on one end-user computer. Compared with other H.264 encoders, it provides the highest quality with the most advanced psychovisual optimizations available. x264 also supports features which are used by different applications such as web videos, Blu-ray, low-latency video applications and television broadcasts. Many web video services such as YouTube, Vimeo, Hulu and Facebook make use of x264. x264 is also used by ISPs and television broadcasters.