to decide something beforehand: to mark out or settle in advance what will happen
The word is built from a piece meaning "before" and a verb meaning "to set a boundary" or "to fix a limit." So at its root it pictures God drawing a line ahead of time around something, settling it before it ever happens. In the New Testament God is always the one doing it, and it always points back to a decision made before the world began.
What God settles in advance is not the same in every verse. Sometimes it is an event, such as the death of Jesus, or a plan, such as the hidden wisdom of the cross. Sometimes it is people and where they are headed, such as being shaped to resemble Jesus, being adopted as God's children, or receiving a promised inheritance. The two senses below sort the occurrences along that line: what is fixed in advance is either a happening or a plan, or else it is people and their destiny.
Because the verb only ever names a settled, before-time decision, that core holds across all six places. What scholars argue about is not what the word means but who or what it covers and how it fits with human freedom, and those debates are noted below rather than split into separate meanings.