What does alternate mean?
The definition of alternate
Al*ter”nate, a.
Etym: [L. alternatus, p. p. of alternate, fr. alternus. See Altern, Alter.]

1 Being or succeeding by turns; one following the other in succession of time or place; by turns first one and then the other; hence, reciprocal. And bid alternate passions fall and rise. Pope.
2 Designating the members in a series, which regularly intervene between the members of another series, as the odd or even numbers of the numerals; every other; every second; as, the alternate members 1, 3, 5, 7, etc. ; read every alternate line.
3 (Bot.)
Distributed, as leaves, singly at different heights of the stem, and at equal intervals as respects angular divergence. Gray. Alternate alligation. See Alligation.

— Alternate angles (Geom.), the internal and angles made by two lines with a third, on opposite sides of it. It the parallels AB, CD, are cut by the line EF, the angles AGH, GHD, as also the angles BGH and GHC, are called alternate angles.
— Alternate generation. (Biol.) See under Generation.

Al*ter”nate, n.
1 That which alternates with something else; vicissitude. [R.] Grateful alternates of substantial. Prior.
2 A substitute; one designated to take the place of another, if necessary, in performing some duty.
3 (Math.)
A proportion derived from another proportion by interchanging the means.
Al”ter*nate, v. t.
[imp. & p. p. Alternated; p. pr. & vb. n.Alternating.]
Etym: [L. alternatus, p. p. of alternare. See Altern.]

To perform by turns, or in succession; to cause to succeed by turns; to interchange regularly. The most high God, in all things appertaining unto this life, for sundry wise ends alternates the disposition of good and evil. Grew.

Al”ter*nate, v. i.
1 To happen, succeed, or act by turns; to follow reciprocally in place or time; — followed by with; as, the flood and ebb tides alternate with each other. Rage, shame, and grief alternate in his breast. J. Philips. Different species alternating with each other. Kirwan.
2 To vary by turns; as, the land alternates between rocky hills and sandy plains.