Latin for 'mother of reading'; plural matres lectionis. A term used in Hebrew grammar (possibly a direct translation of a native Hebrew term - anyone?) to indicate a letter used not as a consonant but to indicate a vowel.
This is nonsense in fully alphabetic languages like English or Greek, where vowels are letters, but in the Semitic languages, from which our alphabet is derived, the basic principle is only to write consonants. However, in both Hebrew and Arabic - and possibly in other Semitic languages but I can't speak for them with any knowledge - long vowels are written using the nearest semivowel consonant, i.e. long î is written iy and long û is written uw.
In Arabic long â is written with the glottal stop, as a?. However I have never actually seen the term matres lectionis used in reference to Arabic, though the principle is the same as in Hebrew. In classical Arabic there are only these three vowels. Hebrew has a more complex vowel system: â was written ah, ê was written ey, ô was written ow, and it would be needlessly distracting to get more detailed than that.
One difference is that in Arabic the use of consonants is always exactly correlated with vowel length (with the exception of a tiny handful of spelling irregularities), but in Hebrew this is not the case. Ancient Hebrew is usually taught as if the group iy was pronounced longer than plain i, but hard evidence does not support this. There quite possibly were long and short vowels in spoken Biblical Hebrew, but they can not be reconstructed with certainty from the fact that almost a thousand years later the Masoretes put that interpretation on the sacred consonant text to which they were adding vowel diacritics.