In various places the Torah refers to מַלְאַךְ ה׳; e.g., twice in Bereishis 22. Mechon Mamre’s JPS translation renders this as “the angel of the LORD”, but is the definite article implicit in this form of the word מלאך, or should it be translated as “an angel of the LORD”?
… which is not the way I think of it, and I'm not sure it's a good way of thinking of it, but maybe. Anyway, let me explain:
A noun phrase in Hebrew sometimes consists of a construct noun and another noun following. The construct noun is something like "מַלְאַךְ" and the noun following it can be "ה׳". In that case, the first, construct noun is definite, i.e. it means "the [something] of", here "the messenger of" or "the angel of". (There are exceptions.)
Note, though, that the definiteness can mean various things. In particular, it need not mean that the messenger/angel under consideration is God's only messenger/angel. (This is like in English, where we say "give him the green folder": there's certainly more than one green folder, but only one under consideration at the moment.) In Genesis 22, the first "מַלְאַךְ ה׳" can't easily be read as referring to a specific messenger/angel mentioned earlier in the text, so the definiteness must be weaker, perhaps something like "the specific messenger/angel that God sent this time". Perhaps some of the commentators there discuss this, but that wasn't your question.