Noun

1. The reciprocal succession of (normally two) things in time or place; the act of following and being followed by turns; alternate succession, performance, or occurrence; as, the alternation of day and night, cold and heat, summer and winter, hope and fear.
2. The response of the congregation speaking alternately with the minister.
3. ablaut
4. The "inclusive or" truth function.
5. A sequence that alternates between positive and negative values. (Sometimes wrongly used to mean a permutation.)