The Collaborative International Dictionary of English v.0.48:

Es \Es\ n.
   the chemical symbol for einsteinium, a transuranic element
   with atomic number 99. The atomic weight of the longest-lived
   isotope, with a half-life of 276 days, is 254. The first
   isotope discovered, having atomic weight 253 and a half-life
   of 20 days, was recognized in 1952 in the debris from a
   hydrogen bomb test. As much as 3 micrograms of einsteinium
   were produced by a complex process involving long irradiation
   of plutonium isotopes in nuclear reactors. Its chemical
   properties are those of a trivalent actinide element. --HCP61

   Syn: einsteinium, atomic number 99.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

einsteinium
    n 1: a radioactive transuranic element produced by bombarding
         plutonium with neutrons [syn: einsteinium, Es, E,
         atomic number 99]

The Elements (07Nov00):

einsteinium
Symbol: Es
Atomic number: 99
Atomic weight: (254)
Appearance is unknown, however it is most probably metallic and silver
or gray in color. Radioactive metallic transuranic element belonging to
the
actinoids. Es-254 has the longest half-life of the eleven known isotopes
at
270 days. First identified by Albert Ghiorso and associates in the
debris of
the 1952 hydrogen bomb explosion. In 1961 the first microgram quantities
of
Es-232 were separated. While einsteinium never exists naturally, if a
sufficient amount was assembled, it would pose a radiation hazard.