alternate

from The American Heritage® Dictionary of the English Language, 5th Edition.

• intransitive verb To occur in a successive manner.
• intransitive verb To act or proceed by turns.
• intransitive verb To pass back and forth from one state, action, or place to another.
• intransitive verb Electricity To reverse direction at regular intervals in a circuit.
• intransitive verb To do or execute by turns.
• intransitive verb To cause to alternate.
• adjective Happening or following in turns; succeeding each other continuously.
• adjective Designating or relating to every other one of a series.
• adjective Serving or used in place of another; substitute.
• adjective Arranged singly at each node, as leaves or buds on different sides of a stem.
• adjective Arranged regularly between other parts, as stamens between petals.
• noun A person acting in the place of another; a substitute.
• noun An alternative.

from The Century Dictionary.

• To do or perform by turns, or in succession.
• To cause to succeed or follow one another in time or place reciprocally; interchange reciprocally.
• To follow one another in time or place reciprocally: generally followed by with: as, the flood and ebb tides alternate one with the other.
• To pass from one state, action, or place to a second, back to the first, and so on indefinitely: used with between, and sometimes with from: as, he alternates between hope and despair, or from one extreme to another; the country alternates between woods and open fields.
• In elect, same as alternating.
• Being by turns; following each the other, recurringly, in succession of time or place; hence, reciprocal.
• Specifically, in botany: Placed at unequal heights upon the axis: as, alternate leaves, which are solitary at the nodes, in distinction from opposite or verticillate.
• Opposite to the intervals between organs: as, petals which are alternate with sepals, or stamens with petals.
• Belonging to a series between the two members of every pair in which a member of another series intervenes; having one intervening between the two of each pair; every second: as, to read only the alternate lines; the odd numbers form one series of alternate numerals, the even numbers another.
• Consisting of alternating parts or members; proceeding by alternation: as, an alternate series; alternate riming; alternate proportion.
• In cryptogamic bot, the passage of a plant through a succession of unlike generations before the initial form is reproduced. Usually the succession is one in which one sexually produced form alternates with another produced asexually. The alternation of those sexually produced may be with those parthenogenetically produced (heterogenesis or heterogamy, which see), or with those produced by budding (metagenesis).
• noun That which happens by turns with something else; vicissitude.
• noun In political conventions and some other representative bodies, one authorized to take the place of another in his absence; a substitute.

from the GNU version of the Collaborative International Dictionary of English.

• adjective 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.
• adjective 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; ; read every alternate line.
• adjective (Bot.) Distributed, as leaves, singly at different heights of the stem, and at equal intervals as respects angular divergence.
• adjective See Alligation.
• adjective (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.
• adjective (Biol.) See under Generation.
• intransitive verb To happen, succeed, or act by turns; to follow reciprocally in place or time; -- followed by with.
• intransitive verb To vary by turns.
• noun rare That which alternates with something else; vicissitude.
• noun A substitute; one designated to take the place of another, if necessary, in performing some duty.
• noun (Math.) A proportion derived from another proportion by interchanging the means.
• transitive verb To perform by turns, or in succession; to cause to succeed by turns; to interchange regularly.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective 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.
• adjective mathematics 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.
• adjective US Other or alternative.
• adjective botany Distributed, as leaves, singly at different heights of the stem, and at equal intervals as respects angular divergence. --Gray.
• noun That which alternates with something else; vicissitude.
• noun A substitute; an alternative; one designated to take the place of another, if necessary, in performing some duty.
• noun mathematics A proportion derived from another proportion by interchanging the means.
• noun US A replacement of equal or greater value or function.
• noun heraldry Figures or tinctures that succeed each other by turns.
• verb transitive To perform by turns, or in succession; to cause to succeed by turns; to interchange regularly.