The Collaborative International Dictionary of English v.0.48:
H \H\ ([=a]ch),
the eighth letter of the English alphabet, is classed among
the consonants, and is formed with the mouth organs in the
same position as that of the succeeding vowel. It is used
with certain consonants to form digraphs representing sounds
which are not found in the alphabet, as sh, th, [th], as in
shall, thing, [th]ine (for zh see [sect]274); also, to modify
the sounds of some other letters, as when placed after c and
p, with the former of which it represents a compound sound
like that of tsh, as in charm (written also tch as in catch),
with the latter, the sound of f, as in phase, phantom. In
some words, mostly derived or introduced from foreign
languages, h following c and g indicates that those
consonants have the hard sound before e, i, and y, as in
chemistry, chiromancy, chyle, Ghent, Ghibelline, etc.; in
some others, ch has the sound of sh, as in chicane. See Guide
to Pronunciation, [sect][sect] 153, 179, 181-3, 237-8.
Note: The name (aitch) is from the French ache; its form is
from the Latin, and this from the Greek H, which was
used as the sign of the spiritus asper (rough
breathing) before it came to represent the long vowel,
Gr. [eta]. The Greek H is from Ph[oe]nician, the
ultimate origin probably being Egyptian. Etymologically
H is most closely related to c; as in E. horn, L.
cornu, Gr. ke`ras; E. hele, v. t., conceal; E. hide, L.
cutis, Gr. ky`tos; E. hundred, L. centum, Gr.
"e-kat-on, Skr. [.c]ata.
H piece (Mining), the part of a plunger pump which contains