The full form of H265 is H.265 OR High Efficiency Video Coding

H.265 (also known as High Efficiency Video Coding, or HEVC) is the latest in a long line of video compression standards. Like its predecessor, H.264, H.265 was initially developed for the broadcast industry by the ITU-T Video Coding Experts Group and the ISO/IEC Moving Pictures Experts Group.

However, as good as H.264 is for SD and HD resolutions, its ability to evolve to handle the demands of the latest high-resolution imaging formats, such as 4K, is limited since 4K stored in H.264 creates a proportionally larger file. H.265 expands on the efficiencies of the H.264 compression standard by generating the highest-quality images in the smallest video file possible.

H.265 essentially offers the same level of picture quality as H.264, but with more efficient codec, so there’s less data to manage. H.265 uses only half the bit rate of H.264. It significantly reduces bandwidth and storage requirements which effectively lowers infrastructure costs and ultimately makes high-resolution surveillance systems more affordable. Decoding H.265 streams requires more computational power when viewing. However, most VMS systems easily leverage GPU accelerated decoding power available from off-the-shelf video cards to achieve the required performance at little to no extra cost. When recording H.265, the NVR simply sees a smaller file to be stored since no decoding is necessary until playback is required.

see also What does HEVC mean? What is the full form of HEVC?