Computing

• High definition display, for high-definition video, high-definition television (HDTV), or Intel High Definition Audio
• Hard disk drive, a computer device which stores digitally encoded data
• /dev/hdd, the slave device on the second ATA channel

Other

• Heating degree day
  Heating degree day
  Heating degree day is a measurement designed to reflect the demand for energy needed to heat a home or business. It is derived from measurements of outside air temperature. The heating requirements for a given structure at a specific location are considered to be directly proportional to the...
  
  , quantitative indices designed to reflect the demand for energy
• Horizontal Directional Drilling, a steerable trenchless method of installing underground pipes