The Collaborative International Dictionary of English v.0.48:

Er \Er\ n.
   the chemical symbol for erbium, a rare earth element. It
   has atomic number 68 and an atomic weight of 167.26.

   Syn: erbium.
        [WordNet 1.5 +PJC]

The Collaborative International Dictionary of English v.0.48:

-er \-er\
   .

   1. [AS. -ere; akin to L. -arius.] The termination of many
      English words, denoting the agent; -- applied either to
      men or things; as in hater, farmer, heater, grater. At the
      end of names of places, -er signifies a man of the place;
      as, Londoner, i. e., London man.
      [1913 Webster]

   2. [AS. -ra; akin to G. -er, Icel. -are, -re, Goth. -iza,
      -?za, L. -ior, Gr. ?, Skr. -[imac]yas.] A suffix used to
      form the comparative degree of adjectives and adverbs; as,
      warmer, sooner, lat(e)er, earl(y)ier.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Er
    n 1: a trivalent metallic element of the rare earth group;
         occurs with yttrium [syn: erbium, Er, atomic number
         68]
    2: a room in a hospital or clinic staffed and equipped to
       provide emergency care to persons requiring immediate medical
       treatment [syn: emergency room, ER]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

ER
       Enterprise Replication (IDS, Informix, DB)


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

ER
       Externer Rechner (T-Online)


The Free On-line Dictionary of Computing (30 December 2018):

er

    The country code for Eritrea.

   (1999-01-27)


The Free On-line Dictionary of Computing (30 December 2018):

ER

   Entity-Relationship


Hitchcock's Bible Names Dictionary (late 1800's):

Er, watchman