HP may refer to:

• Hewlett-Packard, a former technology corporation that split into two separate companies in 2015:
  • Hewlett Packard Enterprise, an enterprise-focused product and service organization
  • HP Inc., Hewlett-Packard's computer and printer business

HP1 or variant, may refer to:

• Heterochromatin Protein 1, an important marker molecule in epigenetic research
• The postal code for part of Hemel Hempstead in Dacorum, see HP postcode area
• hP1, a Pearson symbol
• Harry Potter and the Philosopher's Stone, the first novel in J.K. Rowling's Harry Potter series
• Harry Potter and the Philosopher's Stone (film), the first film in J.K. Rowling's Harry Potter series
• Handley Page Type A aka H.P.1, an airplane
• HP-1, a glider designed by Richard Schreder
• HP1, a type of photographic stock, see Ilford HP
• Haemophilus phage HP1, a virus
• Hp.1 (proceed), from German railway signalling

Horsepower (hp) is a unit of measurement of power (the rate at which work is done). There are many different standards and types of horsepower. The term was adopted in the late 18th century by Scottish engineer James Watt to compare the output of steam engines with the power of draft horses. It was later expanded to include the output power of other types of piston engines, as well as turbines, electric motors and other machinery. The definition of the unit varied between geographical regions. Most countries now use the SI unit watt for measurement of power. With the implementation of the EU Directive 80/181/EEC on January 1, 2010, the use of horsepower in the EU is permitted only as a supplementary unit.