NOUN (3)

1. a resident of a coastal area;

2. someone who coasts;

3. a covering (plate or mat) that protects the surface of a table (i.e., from the condensation on a cold glass or bottle);

The Collaborative International Dictionary of English v.0.48:

Coaster \Coast"er\, n.
   1. A vessel employed in sailing along a coast, or engaged in
      the coasting trade.
      [1913 Webster]

   2. One who sails near the shore.
      [1913 Webster]

WordNet (r) 3.0 (2006):

coaster
    n 1: a resident of a coastal area
    2: someone who coasts
    3: a covering (plate or mat) that protects the surface of a
       table (i.e., from the condensation on a cold glass or bottle)

The Jargon File (version 4.4.7, 29 Dec 2003):

coaster
 n.

    1. Unuseable CD produced during failed attempt at writing to writeable or
    re-writeable CD media. Certainly related to the coaster-like shape of a CD,
    and the relative value of these failures. ?I made a lot of coasters before
    I got a good CD.?

    2. Useless CDs received in the mail from the likes of AOL, MSN, CI$,
    Prodigy, ad nauseam.

    In the U.K., beermat is often used in these senses.