Definiteness

Definiteness is the property of an ism that tells us if the ism is referring to something specific (مَعْرِفَة) or general — non-specific (نَكِرَة). So an ism has to be either definite (D) or indefinite (I). It is similar to English.

Every ism is indefinite (نَكِرَة) unless it falls into one of the seven categories of definite in Arabic. Say رَجُلٌ and you have said “some man — any man”: the word ranges over its whole kind. A practical test: if الْ will fit on it (رَجُل → الرَّجُل), it was a nakira. In the Quran that choice is deliberate — a nakira can leave someone unnamed, or magnify: عَذَابٌ عَظِيمٌ, “a punishment” beyond naming.

Learn the set, not the words one by one. Every definite ism in Arabic is definite through one of these seven.

Tanwīn is often taken to mean an ism is indefinite. It does not. Many definite isms are proper nouns and they carry tanwīn — مُحَمَّدٌ, حَامِدٌ, زَيْدٌ are all definite. Why? Because they are proper nouns. So tanwīn is not a sign of being indefinite. This is a common misunderstanding.

The word مَعْرِفَة (ma'rifa) in everyday Arabic means knowledge — knowing a thing for what it is.

عَرَفَ الطَّرِيقَ ('arafa al-ṭarīqa) — “he knew the road”

As a grammatical term it means an ism that points at something specifically identified, through one of seven means.

Is every word with ال definite?

Every noun with ال is definite, but not every definite noun has ال — pronouns, names, demonstratives, relative nouns and mudaf constructions are all definite without it.