A synecdoche wherein expression of the edges that encompasses the whole.
Examples abound, but from modern usage: "to search high and low" means to search everywhere.
The Old Testament
Merisms include Genesis 1:1 "heavens and the earth (the entire universe), Psalm 139's God knows "my downsitting and mine uprising" (all my actions), and Genesis 1:5's "evening" and "morning" (one day).
Cyrus H. Gordon's theory - "good and evil"
Gordon (he of the delightful if controversial theory of 'Helleno-Semitica' - a common Semitic heritage of both Hebrew and Helenic Greek cultures) suggests that the biblical phrase “good and evil” ( טוֹב וָרָע ) is a merism. The snake says that if they eat the fruit their eyes will open and they will become like gods, knowing good and evil, which lends credence to this theory.
The New Testament
Paul's merisms in Romans 8:38-39 "death-life (all of human existence and experience); angels-demonic powers (all of the supernatural); present-future (all time); and height-depth (the whole cosmos). Paul intends us to understand that nothing in natural human experience, nothing in the supernatural realm, nothing in time, nothing in space, nothing exists that can separate the Christ one from the Christ, for their nature is one and inseparable." Source
Of course, not every binary set is a merism.