Data that provides information about other data.

Metadata is electronic data that somehow describes an electronic file or its contents, and is usually included in the file itself. An important use for metadata is for searching. A piece of metadata might identify the file, its size, the date it was compiled, its nature, and so on. Metadata is particularly important for making pictures searchable; since a picture of a landscape in the Southwest, for example, can't be “read” by a search engine, data tags such as “ Southwest,” “mesa,” and “arroyo” might be included in the digitized image file. The same can be done for audio files; the tags for a speech might read “Gore,” “climate,” and “Copenhagen.” Metadata tags for a Web page, including tags identifying its most important content, ensure that the page won't be overlooked by a search engine.

*Metaphor* comes from a Greek word meaning “transfer” (or, to stay close to its roots, “carry beyond”). Thus, a metaphor transfers the meaning of one word or phrase to another. Metaphors often include a form of the verb *be* (as in the examples above), and they're often contrasted with similes, which are usually introduced by *like* or *as* (“O, my luve's like a red, red rose”). But, they don't have to include *be*; when you say that the teacher gave us a mountain of homework or that we're drowning in paperwork, these too are *metaphorical* statements.

The part of philosophy having to do with the ultimate causes and basic nature of things.

Just as *physics* deals with the laws that govern the physical world (such as those of gravity or the properties of waves), metaphysics describes what is beyond physics—the nature and origin of reality itself, the immortal soul, and the existence of a supreme being. Opinions about these *metaphysical* topics vary widely, since what's being discussed can't be observed or measured or even truly known to exist. So most metaphysical questions are still as far from a final answer as they were when Plato and Aristotle were asking them.

At first glance, *metaphor* and *metonymy* seem close in meaning, but there are differences. In a metaphor we substitute one thing for something else that's usually quite different; for example, *Web* for a worldwide network of linked computers and their technology. In metonymy, we replace one word or phrase (such as “stock market” or “local government officials” in the examples above) with another word or phrase associated with it. Most familiar *metonyms* are place-names, such as *Hollywood* for “the film industry,” or *K Street* for “Washington lobbyists.” But saying “the press” to refer to the news media, or “sweat” to refer to hard work, could also be called metonymy.

(1) To trickle or filter through something porous. (2) To become spread through.

*Percolate* comes from a Latin verb meaning “to put through a sieve.” Something that percolates filters through something else, just as small particles pass through a sieve. Water is drawn downward through the soil, and this *percolation* usually cleans the water. A slow rain is ideal for percolating into the soil, since in a violent rainstorm most of it quickly runs off. For this reason, drip irrigation is the most effective and water-conserving form of irrigation. Percolation isn't always a physical process; awareness of an issue may percolate slowly into the minds of the public, just as Spanish words may gradually percolate into English, often starting in the Southwest.

To spread through all parts of something.

*Pervade* can be used to describe something physical: a chemical odor may pervade a building, for example, and most scientists believe that outer space is pervaded by mysterious “dark matter.” But the word usually doesn't refer to anything that could be detected by scientific instruments. Thus, humor may pervade a novel, gloom may pervade a gathering, and corruption may pervade a government. And something *pervasive* exists in every part of something: fatherlessness may be a pervasive problem in poor neighborhoods, for instance, and pervasive optimism sometimes causes the stock market to soar.

(1) To spread throughout. (2) To pass through the pores or small openings of.

*Permeate* is often a synonym for *pervade*. We could say, for example, that at exam time the campus is either “permeated” or “pervaded” by a sense of dread. But the two words aren't identical. For one thing, *permeate* can mean simply “pass through,” and is often used when talking about liquids; thus, a boot can be permeated by water, though certain oils make leather less *permeable,* and you might just want to buy boots made of *impermeable* material. And things may “pass through” in a nonphysical way as well; so you might say that anxiety about climate change has started to permeate into the public's consciousness—but once anxiety has become *pervasive* it's pretty much taken over.

To keep at something in spite of difficulties, opposition, or discouragement.

The early settlers of the New World persevered in the face of constant hardship and danger. The Pilgrims of Plymouth Plantation lost half their number in the first winter to disease and hunger, but their *perseverance* paid off, and within five years their community was healthy and self-sufficient. Perhaps more remarkable are all the solitary inventors who have persevered in pursuing their visions for years, lacking any financial support and laughed at by the public.