(1) A layer of a substance, especially one of a series of layers. (2) A level of society made up of people of the same rank or position.

In geology, a stratum is a layer of rock or soil that is distinct from those above and below it. Rock and soil *strata* (notice the plural form) can be seen in road cuts, cliffs, quarries, riverbanks, and sand dunes, and in pieces of limestone, slate, and shale. Archaeologists digging in historical sites are careful to note the stratum where each artifact is found. Earth scientists divide the earth's atmosphere into strata, just as oceanographers divide the ocean's depths into strata. And for social scientists, a stratum is a group of people who are similar in some way, such as education, culture, or income.

The process or state of being formed, deposited, or arranged in layers.

If you look for it, you'll find stratification almost everywhere. On a tall rain- forest tree, there may be different air plants clinging to it, different insects crawling on it, and different mammals making their homes at different levels. The earth beneath you may be *stratified* into several distinctive layers within the first 20 feet. If the wind you're feeling is moving at 10 miles per hour, at 30 feet above your head it may be 20 mph, and in the jet stream above that it may be 150 mph. If you climb a high mountain in Himalayas, you may begin in a lush, wet forest and end up in a windswept environment where not even lichen will grow.

(1) An underlying layer. (2) The base on which an organism lives.

With its Latin prefix *sub-*, “below” (see SUB), *substrate* obviously refers to a layer under something else. Rock may serve as the substrate for the coral in a coral reef. Tiny wafers of silicon (or another semiconductor) serve as the substrate for computer chips. *Substrate* may also mean *subsoil*—that is, the layer under the topsoil, lacking in organic matter or humus. *Substrate* is part of the vocabulary of various other sciences, including chemistry and biology. But although it's mostly a scientific term, writers may also use it to mean simply “foundation”—for instance, when observing that reading is the substrate on which most other learning is based.

A low-lying cloud formation appearing as extensive and often dark horizontal layers, with tops rounded into large balls or rolls.

When a cloud type forms a broad “layer” over the earth, the *strat-* root shows up in its scientific name. The type called simply *stratus* forms a low layer of gray extending over a large area. *Cirrostratus* (“curl layer”) clouds form a high, thin layer often covering the entire sky (but without the wispy curls of ice crystals that give pure *cirrus* clouds their name). *Altostratus* (“high layer”) clouds form a darkish gray mid-altitude layer. *Nimbostratus* (“rainstorm layer”) clouds form a low, dark layer of gray cloud that usually produces light but continuous rain, snow, or sleet (but not violent storms of the kind that give pure *nimbus* clouds their name). *Cumulus* (“heap”) is the familiar puffy fair-weather type of cloud; stratocumulus is its more wintry version, which spreads out in a fairly flat layer, much less “heaped up,” and sometimes dense enough to cover almost the whole sky.

Of or relating to the side.

Lateral shows up in all kinds of contexts. A lateral job change is one that keeps you at about the same job level and salary. A coach might have special drills to improve his players' lateral speed and agility. The British speak of “lateral thinking,” thinking that grabs ideas that may not seem to be relevant but turn out to work well—what we might call “thinking outside the box.” But we know *lateral* best from football. A lateral pass is a pass of the ball between teammates that usually goes to the side and slightly backward from the direction in which they're advancing; unlike a forward pass, a lateral may be made from any position, and any number may be made in a single play.

Involving two groups or countries.

Since the prefix *bi-* means “two” in Latin (see BI/BIN), *bilateral* means essentially “two-sided.” In the days when there were two superpowers, the U.S. and the Soviet Union regularly engaged in bilateral arms negotiations; such negotiations are much less common today. Sometimes *bilateral* refers to two sides of the same thing. A bilateral hip replacement, for instance, replaces both hip bones in the same operation. And *bilateral symmetry* (a term often used by biologists) refers to the fact that, in many organisms (such as humans), the left side is basically the mirror image of the right side.

(1) Associated but of secondary importance. (2) Related but not in a direct or close way.

If an official talking about some policy refers to a collateral issue, he or she means something that may be affected but isn't central to the discussion. To an anthropologist, your cousin would be called a collateral relative, since he or she (unlike your grandmother, brother, or daughter) is “off to the side” of your direct line of descent. As a noun, *collateral* means something provided to a lender as a guarantee of repayment. So if you take out a loan or mortgage to buy a car or house, the loan agreement usually states that the car or house is collateral that goes to the lender if the sum isn't paid.

Having all sides or faces equal.

Since *equi-* means “equal” (see EQU), the meaning of *equilateral* is easy to guess from its roots. The word is mostly used in geometry. The standard polygons (many-sided geometrical shapes)—the pentagon, hexagon, octagon, etc.—are assumed to be equilateral if we don't say otherwise; an equilateral rectangle has the special name *square*. But triangles are particularly important, and many triangles are not equal-sided. The standard polyhedrons (many-sided solids) are also equilateral. Most common is the cube, all of whose sides are square. The tetrahedron has four triangular sides and thus is a pyramid with a triangular base, unlike the pyramids of Egypt with their square bases.