carry
The 1 that moves into the next column when a column's digits add up to too much; at the top of a fixed width, it is the bit that falls off.
In positional addition a carry passes from column i to column i+1 when the column sum reaches the base; the carry out of an n-bit addition is bit n of the full sum, 1 exactly when the unsigned result wrapped around.
Examples
Adding the 4-bit values
1001+1000gives1 0001: the fifth bit is the carry out.
See also