hydrogen.noun
a hydrogen atom is a single proton orbited by an electron: atomic number 1; atomic weight 1.00797; melting point 259.14°C; boiling point 252.8°C; density at 0°C 0.08987 gram per liter; valence 1; it's a colorless, odorless gaseous chemical element having three isotopes, the lightest of all known substances; the perfect balance of abundances of hydrogen and helium in the Universe, also denotes a balance also in the gravitational and weak-nuclear energies. One hydrogen atom holds on to its heat, but two of them in a molecule-dissipate thermal energy. Such design, and so tiny that 10 million of them can span the diameter of the head of a pin!

hydrogen peroxide.noun
a colorless, heavy, strongly oxidizing liquid, H₂O₂, capable of reacting explosively with combustibles and used principally in aqueous (water) solution as a mild antiseptic

hinder, hindered, hindering, hinders.verbs
transitive use-to be or get in the way of; to obstruct or delay the progress of; to make difficult for; thwart; impede; frustrate; to keep or hold back; restrain
intransitive use-to interfere with action or progress
hinder.adjective
the rear position; posterior; hind
hinderer.noun
hindrance.noun
the act of hindering; the condition of being hindered; one that hinders; an impediment; obstacle

hexagon.noun
a polygon having six sides
hexagonal.adjective
hexagonally.adverb

homeostasis.noun
a relatively stable state of equilibrium or a tendency toward such a state between the different but interdependent elements or groups of elements of an organism, population, or group
homeostatic.adjective

hyperons
any baryon that does not decay (except the proton or neutron). The heavier hyperons are lambda, sigma, xi, and omega particles.

homologous.adjective
having a related or similar position, structure, etc.

horrendous.adjective
perfectly horrid; dreadful (the tax rate was horrendous) 
horrendously.adverb

hydrodynamics.noun.plural-but singular in construction 
a branch of physics that deals with the motion of fluids and the forces acting on solid bodies immersed in fluids and in motion relative to them – compare hydrostatics 
hydrodynamicist.noun

hackneyed.adjective
used so often so as to be trite, dull and stereotyped
hackney.transitive verb,-usually passive
to make commonplace and banal by too frequent use

harebrained.adjective
rash, giddy, flighty

helix.noun
a helix's looping curve is identical to the one below or above. Helixes are a form of, but different from spirals;-example

heliosphere.noun
the region in space through which the sun's gases and magnetic field extend.

hoodwink, hoodwinked, hoodwinking, hoodwinks.transitive verbs
to take in by deceptive means; deceive; bamboozled
hoodwinker.noun

hertz.noun-and.plural,.abbreviation-Hz 
a unit of frequency equal to one cycle per second (after Heinrich Rudolf Hertz, 1857-1894; German physicist who was the first to produce radio waves artificially); a kilohertz thusly is one thousand hertz (1000 Hz)

hitherto.adverb
until this time; (the weather, which had hitherto been sunny and mild, suddenly turned cold)

hither.adverb
to or toward this place (reach in hither)
hither.adjective
located on the near side
idiom.hither and thither or hither and yon; in or to many places; here and there (looked hither and thither for the ring; ran hither and yon)