an allotted, inalienable share: the permanent portion (most often a piece of land) that is assigned to someone and stays bound to them and their family line
At its core this word names a portion that is given, kept, and passed on. A nachalah is not just property a person happens to own: it is a share that has been assigned, most often a tract of land, and that is meant to stay with the family or tribe it belongs to. That joining of two ideas, a portion handed out and a portion that cannot simply be sold off, is what holds all the uses together. A man is buried on his nachalah (Joshua 24:30); a woman's right to receive one is worth a ruling from God himself (Numbers 27:7); and Naboth refuses to give his up even to a king, because it is his fathers' nachalah and he is bound to it.
This sense of an assigned, identity-forming holding makes the word natural for the bond between God and his people, and the Bible runs it in both directions at once. Israel is God's nachalah, the people he marked out as his own portion (Deuteronomy 4:20, Psalm 33:12). At the same time the land Israel receives is the nachalah God grants them (Deuteronomy 4:21). And the priestly tribe, who get no land at all, are told that the LORD himself is their nachalah (Numbers 18:20, Deuteronomy 18:2). The same word stretches across all of these because the underlying picture is one: a portion assigned and held within a lasting relationship.
From there the word reaches into other settings. A father's estate handed to his children is a nachalah (Proverbs 19:14); so, surprisingly, are children themselves, named as a portion that comes from God rather than something a person achieves (Psalm 127:3). In the book of Job the friends even use it for the grim portion of ruin that they say is allotted to the wicked. Most poignantly, the prophets use it to name what is at stake when the bond breaks: Jeremiah mourns that God's own nachalah, his people, has turned against him (Jeremiah 12:7-9), and Lamentations cries that Israel's nachalah has been handed to strangers (Lamentations 5:2). The loss is never merely material; it is the unraveling of a relationship.