Merriam-Webster's Word of the Day for April 19, 2012 is:

threshold • \THRESH-hohld\  • noun
1 : the section of wood or stone that lies under a door 2 a : the means or place of entry : entrance b : the place or point of beginning 3 : the point or level at which a physical or mental effect begins to be produced

Examples:
She fell in love with the old house from the moment she first stepped across the threshold.

"The protracted delegate fight has raised the possibility that none of the three contenders will reach the threshold needed to secure the nomination before the Republican National Convention in Tampa, Fla., in August." — From an article by Michael Finnegan and John Hoeffel in the Chicago Tribune, March 14, 2012

Did you know?
The earliest known use of "threshold" in the English language is from Alfred the Great's Old English translation of the Roman philosopher Boethius's De consolatione philosophiae. In this translation, which was written around 888, "threshold" appears as "þeorscwold" (that first letter is called a thorn and it was used in Old English and Middle English to indicate the sounds produced by "th" in "thin" and "this"). The origins of this Old English word are not known, though it is believed to be related to Old English "threscan," from which we get the words "thresh," meaning "to separate seed from (a harvested plant) using a machine or tool" and "thrash," meaning, among other things "to beat soundly with or as if with a stick or whip."