chosen: singled out and picked from among others, above all those God has selected as his own
This word is the adjective for someone or something that has been picked out from a larger group. In everyday Greek it could describe choice, first-rate goods, but in the New Testament it almost always points to God as the one doing the picking, and to the people he has picked.
By far its most common use is as a plural noun, the chosen ones: God's people, those he has set his love on and claimed for himself. In the teaching about the last days, they are the ones God will protect and gather from the ends of the earth. In the letters they are simply another name for believers, the community God has chosen.
A few uses point away from the community to a single figure God has singled out. The clearest is the crowd's taunt at the cross, calling Jesus God's Chosen One, and the picture of Christ as the choice, chosen stone that God lays as a foundation. Twice the word draws a sharp line between being merely invited and being actually chosen, in the saying that many are called but few are chosen. Once it describes the angels God has set apart.
The hard part is not what the word means but the weight readers pile onto it. The Greek adjective simply says picked out; the long arguments about how and why God chooses are theology built on top of the word, not the meaning of the word itself.