1. 1. A barrier constructed across a waterway to control the flow or raise the level of water.
   2. A body of water controlled by such a barrier.
2. A barrier against the passage of liquid or loose material, as a rubber sheet used in dentistry to isolate one or more teeth from the rest of the mouth.
3. An obstruction; a hindrance.

1. To hold back or confine by means of a dam.
2. To close up; obstruct: He tried to dam his grief. See synonyms at hinder¹.

[Middle English.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.