Source : Webster's Revised Unabridged Dictionary (1913)

Ephemeron \E*phem"e*ron\, n.; pl. {Ephemera}. [NL. See
   {Ephemera}.] (Zo["o]l.)
   One of the ephemeral flies.

Ephemera \E*phem"e*ra\, n. [NL., fr. Gr. ? a day fly, fr. ?
   daily, lasting but a day; ? over + ? day.]
   1. (Med.) A fever of one day's continuance only.

   2. (Zo["o]l.) A genus of insects including the day flies, or
      ephemeral flies. See {Ephemeral fly}, under {Ephemeral}.

May \May\, n. [F. Mai, L. Maius; so named in honor of the
   goddess Maia (Gr. ?), daughter of Atlas and mother of Mercury
   by Jupiter.]
   1. The fifth month of the year, containing thirty-one days.
      --Chaucer.

   2. The early part or springtime of life.

            His May of youth, and bloom of lustihood. --Shak.

   3. (Bot.) The flowers of the hawthorn; -- so called from
      their time of blossoming; also, the hawthorn.

            The palm and may make country houses gay. --Nash.

            Plumes that micked the may.           --Tennyson.

   4. The merrymaking of May Day. --Tennyson.

   {Italian may} (Bot.), a shrubby species of {Spir[ae]a} ({S.
      hypericifolia}) with many clusters of small white flowers
      along the slender branches.

   {May apple} (Bot.), the fruit of an American plant
      ({Podophyllum peltatum}). Also, the plant itself
      (popularly called {mandrake}), which has two lobed leaves,
      and bears a single egg-shaped fruit at the forking. The
      root and leaves, used in medicine, are powerfully drastic.
      

   {May beetle}, {May bug} (Zo["o]l.), any one of numerous
      species of large lamellicorn beetles that appear in the
      winged state in May. They belong to {Melolontha}, and
      allied genera. Called also {June beetle}.

   {May Day}, the first day of May; -- celebrated in the rustic
      parts of England by the crowning of a May queen with a
      garland, and by dancing about a May pole.

   {May dew}, the morning dew of the first day of May, to which
      magical properties were attributed.

   {May flower} (Bot.), a plant that flowers in May; also, its
      blossom. See {Mayflower}, in the vocabulary.

   {May fly} (Zo["o]l.), any species of {Ephemera}, and allied
      genera; -- so called because the mature flies of many
      species appear in May. See {Ephemeral fly}, under
      {Ephemeral}.

   {May game}, any May-day sport.

   {May lady}, the queen or lady of May, in old May games.

   {May lily} (Bot.), the lily of the valley ({Convallaria
      majalis}).

   {May pole}. See {Maypole} in the Vocabulary.

   {May queen}, a girl or young woman crowned queen in the
      sports of May Day.

   {May thorn}, the hawthorn.

Source : WordNet®

ephemera
     See {ephemeron}
     [also: {ephemerae} (pl)]

ephemeron
     n : an insect that lives only for a day in its winged form [syn:
          {ephemera}]
     [also: {ephemera} (pl)]

ephemera
     n 1: something transitory; lasting a day
     2: an insect that lives only for a day in its winged form [syn:
         {ephemeron}]
     [also: {ephemerae} (pl)]