What's the Word?

There must be a word—although I do not know what it is—to define a metaphorical term whose meaning has supplanted the literal meaning of the original.  There are plenty of these words in English.  A few examples are:

Mortarboard—Chances are the first thing you think of when you hear this word is a flat academic hat—not a mason’s tool for holding a gooey building material.  Webster’s Collegiate Dictionary, published in 2007,  even lists the academic hat as the primary meaning.  Webster’s New International of 1949, however, says a flat board with a handle for holding mortar should go first, with the hat in second place. I conclude that this shift in primary meaning has taken place over a half-century.

Warhorse—Most people think first of a veteran public official or an overdone artistic work before settling on the original meaning, a horse used in battle.

Beanpole—A tall thin person comes to mind before a wooden stick for a bean vine to grow on.

Blockbuster—Originally this was a bomb so powerful it could blow up a city block, but now it’s primarily a production of some sort that is extravagant and spectacular.

Melting Pot—This term has been used so often to describe the diverse society of the United States that hardly anyone would now think it meant a crucible in which to dissolve substances over high heat.

Barn Burner—Very few barns actually go up in flames when this term is used; instead it refers to something that arouses a great deal of excitement.

Spare Tire—Okay, which do you think of first—that Goodyear radial in the trunk or that extra layer of fat around the tummy?

Nest Egg—Which is it—an egg left in a nest to induce a hen to lay more (original meaning) or a sum of money saved for a rainy day?  I’ll bet you think first of the latter.

I’m sure there must be many more such terms that have virtually lost their original definition and now primarily denote their metaphorical meaning.  Now all I have to do is find out what to call them.

The Bard of Buffalo Bayou (who has been uncharacterstically dormant for a blessed while) has bestirred himself (having thought he heard the tinkle of ice cubes) to regurgitate a few lines of gibberish. 

            A Broadway composer, who thought anything goes,  

            At a college commencement soon started to doze,

            For alas, one address by a man in a mortarboard,

            I have to confess, left poor old Cole Porter bored.