What is Alternation?

Alternation definition and meaning on Dictionary terms:
the act or process of alternating or the state of being alternated.
alternate succession; repeated rotation: the alternation of the seasons.
Electricity. a single fluctuation in the absolute value of an alternating current or voltage from zero to a maximum and back to zero, being equal to one half cycle.
Linguistics. variation in the form of a linguistic unit as it occurs in different environments or under different conditions, as between the -ed and -en forms of the past participle in danced and spoken or between the (t) and (d) pronunciations of the past tense suffix -ed in hopped and rubbed.