H, or h is a consonant in Esperanto orthography, representing a voiceless velar fricative  or voiceless uvular fricative . Its name in Esperanto is ho (pronounced ).

H (named aitch or haitch in Ireland and parts of Australia; plural aitches or haitches) is the eighth letter in the ISO basic Latin alphabet.

H is the eighth letter of the Latin alphabet.

H (minuscule: h) is a letter of the Latin alphabet, derived from H with the addition of a bar. It is used in Maltese for a voiceless pharyngeal fricative consonant (corresponding to the letter heth of Semitic abjads). Lowercase is used in the International Phonetic Alphabet for the same sound.

H+ or h+ may refer to:

H+ is the symbol for the Transhumanist Party with Zoltan Istvan running for U.S. President in 2016

Hadouken! are a British band and was formed in London in 2006 by singer, writer and producer James Smith, alongside his girlfriend, synth player Alice Spooner, with guitarist Daniel "Pilau" Rice, bassist Christopher Purcell and drummer Nick Rice. The band name is taken from the name of the special attack of the same name from the Street Fighter video game series. It was in Leeds that Hadouken! began their own record label, Surface Noise Records.

Hamiltonian mechanics is a theory developed as a reformulation of classical mechanics and predicts the same outcomes as non-Hamiltonian classical mechanics. It uses a different mathematical formalism, providing a more abstract understanding of the theory. Historically, it was an important reformulation of classical mechanics, which later contributed to the formulation of statistical mechanics and quantum mechanics.

In chemistry, a hydride is the anion of hydrogen, H^-, or, more commonly, it is a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, the hydrogen atom is bonded to a more electropositive element or group. Compounds containing hydrogen bonded to metals or metalloid may also be referred to as hydrides, even though in this case the hydrogen atoms can have a protic character. Almost all of the elements form binary compounds with hydrogen, the exceptions being He, Ne, Ar, Kr, Pm, Os, Ir, Rn, Fr, and Ra.

Many programming languages and other computer files have a directive, often called include (as well as copy and import), that causes the contents of a second file to be inserted into the original file. These included files are called copybooks or header files. They are often used to define the physical layout of program data, pieces of procedural code and/or forward declarations while promoting encapsulation and the reuse of code.

Macron below, , is a combining diacritical mark used in various orthographies.

The Planck constant (denoted , also called Planck's constant) is a physical constant that is the quantum of action, central in quantum mechanics.

In mathematics, the quaternions are a number system that extends the complex numbers. They were first described by Irish mathematician William Rowan Hamilton in 1843 and applied to mechanics in three-dimensional space. A feature of quaternions is that multiplication of two quaternions is noncommutative. Hamilton defined a quaternion as the quotient of two directed lines in a three-dimensional space or equivalently as the quotient of two vectors.