HD is an English acronym whose meaning can refer to "High Definition" or "Hard Disk".
The expression "High Definition" means "High Definition". HDTV is a feature of television sets that receive a digital signal and display a high resolution image.
The format of these TVs is wide (16:9), unlike analog TVs that have the 4:3 format.
The HDTV standard is used in televisions HD Ready or Full HD. The difference between the two is the detail of the image. While HD Ready has 1366 x 768 pixels, Full HD reaches 1920 x 1080 pixels, presenting a full image resolution.
The expression "hard disk” stands for “Hard Disk”. The hard disk is the computer's permanent memory and stores all the information that is saved by the user, in addition to the operating system's own applications.
HD is also called “secondary memory”. It is physical, non-volatile memory. It differs from “RAM memory” precisely in that it stores information even when the computer is turned off.
The hard disk is a device with large data storage capacity and is built into the computer.
The external hard drive, connected to the computer through a USB port, is a popular alternative for storing all types of data: music, movies, photos, backup, etc.
Read more about RAM memory.