a testing, a trial: an ordeal that forces out into the open what someone or something is really made of
Deuteronomy uses this noun only in the plural, and only in one spot in the story: Moses reminding a new generation what God did to get them out of Egypt. "By testings, by signs and by wonders, and by war, and by a mighty hand, and by an outstretched arm" (Deuteronomy 4:34). The plagues are being filed under a heading here. They were not random disasters. They were ordeals that forced the question of who actually held power in Egypt.
The word is built on the verb "to put to the test" (נָסָה). A testing in this sense is not a written exam. It is a situation deliberately created so that whatever is really there has to show itself, and in Egypt what showed itself was Pharaoh's stubbornness and the emptiness of the gods behind him, one blow at a time, while Israel watched.
The same idea produced a place name. When Israel quarreled with God over water in the desert, the spot was called Massah (מַסָּה), meaning "Testing", because there the people put God to the test instead (Exodus 17:7). The noun runs in both directions: God tests people, and people test God.