er

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

• interjection Used to express hesitation or uncertainty.

from The Century Dictionary.

• A Middle English form of ere.
• A suffix of Latin origin, denoting usually a person, and often an agent, but not, like -er, usually associated with a verb.
• An English suffix, originally and properly attached to verbs to form nouns of the agent, as in baker, creeper, delver, driver, reader, sower, writer, etc.
• In chem., the symbol for erbium.
• A suffix of certain nouns, mostly technical terms of the law (from Old Law French), as attainder, misnomer, trover, user, non-user, waiver, etc. In endeavor, endeavour, the orig. -er is disguised in the spelling.
• A suffix of verbs, giving them a frequentative and sometimes a diminutive sense, as patter from pat, swagger from swag, flutter from float, sputter from spout, etc.
• In heraldry, an abbreviation of ermine.
• A simplified spelling of err.
• A suffix of adjectives, forming the comparative degree, as in colder, deeper, greater, bigger, etc., and being cognate with the Latin comparative suffix -or, -ior, neuter -us, -ius, represented in English in major, minor, minus, prior, superior, inferior, etc.

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

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

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

• interjection UK Said when hesitating in speech.
• verb informal To utter the word "er" when hesitating in speech, found almost exclusively in the phrase um and er.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a trivalent metallic element of the rare earth group; occurs with yttrium
• noun a room in a hospital or clinic staffed and equipped to provide emergency care to persons requiring immediate medical treatment