I intend to forward the following definitions to the Unicode Book
Committee as improvements on the current definitions:

Abjad. A writing system in which only consonants are indicated. The
Phoenician script is a prototypical abjad; a better-known example is
the Arabic writing system, though it is not a "pure" abjad because
consonant letters like WAW and YEH can be used to mark long vowels
/u:/ or /i:/. Some abjad writing systems optionally indicate vowels
with the use of secondary marks on the consonants. The term "abjad"
is derived from the first four letters of the traditional order of
the Arabic script (ALIF, BEH, JEEM, DAL).

Alphabet. A writing system in which both both consonants and vowels
are indicated. The most widespread and well-known example is the
Latin writing system. The correspondence between letters and sounds
may be either more or less exact. Many alphabets do not exhibit a
one-to-one correspondence between distinct sounds and letters or
groups of letters used to represent them; often this is an indication
of original spellings which were not changed as the language changed.
The term "alphabet" is derived from the first two letters of the
Phoenician script (ALF, BET).

I am satisfied with both of these as being suitable with regard to
the aims of the Unicode glossary, though I would entertain comment
about whether Phoenician ALF, BET is going to be controversial or
not. The alternative is Greek ALPHA, BETA.

I will now turn to the definition of Abugida on a different thread.
--
Michael Everson * * Everson Typography * * http://www.evertype.com