What does alternation mean?
The definition of alternation
Al`ter*na”tion, n.
Etym: [L. alternatio: cf. F. alternation.]

1 The reciprocal succession of 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 (Math.)
Permutation.
3 The response of the congregation speaking alternately with the minister. Mason. Alternation of generation. See under Generation.