a generation: the people alive together in one stretch of time, and by extension the span of time they fill.
At its core this word names a circle of people who share the same lifetime, the contemporaries who live and die together. From that human core it stretches in two directions. Looking at the people, it can name a kind of person: the company that shares a common character, so the Bible can speak of a crooked generation or an upright one, meaning the people marked by that trait.
Looking at time, it names the stretch itself, the lifetime such a group occupies, and then the chain of lifetimes that follow one another. This is why the word so often appears doubled, generation after generation, reaching back to the ancestors and forward to the descendants. In that doubled form it becomes the Bible's ordinary way of saying always or forever from a human vantage point, not by naming endless time directly but by piling up lifetime upon lifetime without end.
A distinct, fixed use settles in the laws of Exodus through Numbers, where the plural "your generations" means your descendants down the line, the future Israel who must keep a given command permanently. There the word has hardened into a near formula for lasting obligation passed from parents to children.
The shades hold together because they all radiate from the same picture: a body of people sharing one span of life. Whether the focus falls on who they are, how long they last, or the line that succeeds them, it is the same idea seen from a different angle.
Nuances
Disputed
Whether a small number of verses preserve a separate noun meaning dwelling or encampment (a homonym), or whether every occurrence belongs to the one word generation/lifetime. The clearest test case is Isaiah 38:12, where the tent image makes "dwelling" attractive, though many read it as "my lifetime is folded up."
- majorityAll occurrences belong to one word, generation/lifetime/age; the apparent dwelling sense is the lifetime sense read within a tent metaphor.; BDB (Brown-Driver-Briggs), s.v. דּוֹר; DCH (Clines, ed.), s.v. דּוֹר
- minorityA separate word meaning dwelling or circle of tents lies behind Isaiah 38:12, on the basis of cognate words for a circular dwelling.; HALOT (Koehler-Baumgartner), s.v. דּוֹר II