What do you call a word that only ever appears as a plural?

Most nouns have a singular form and a plural form, but some nouns in this wacky, illogical, beautiful thing we call the English language have evolved to have only a plural form, even when they refer to a single thing.
It's bird-watching day, so you pull on your favorite bird-watching clothes, including your lucky pants (or shorts, jeans, chinos, slacks, trousers, breeches, or even tights). After you finish getting dressed, you put on your sunglasses, grab your brand new binoculars, and head for the outskirts of town, where the bird-watching is the best. On the way there, you hear news on the radio of an outbreak of bird flu, which leaves you wondering, 'What are the odds that I'll catch it?'

In linguistics, a noun that always appears in a plural form is called a plurale tantum, a Latin phrase that means "always plural." (Its plural form is pluralia tantum.) They're also called, believe it or not, defective nouns.

Pluralia tantum most often take a plural verb, as in the sentence, "These pants are too long, but those shorts are too short." Country names, though, take a singular verb: "The United States exports orange-flavored badgers to The Maldives."

Others can take either a plural or singular verb depending on the situation:

Acoustics shows us how to make these rock bands sound even louder.
The acoustics in this room are amazing!
Physics attempts to explain the rules of the universe.
The physics behind your anti-gravity boots are questionable at best.

Pluralia tantum are not unique to English, either. Almost all languages have some words that are always used as plurals, such as the Italian cappelini (thin pasta), the French mœur (morals), the Dutch hersenen (brains), and the German Eltern (parents).