MEDI comes form the Latin medius, meaning "middle." Our word medium refers to something in a middle position. The medieval period of European history, also known as the Middle Ages, is the period between Greek and Roman antiquity and the "mordern age." But why people around 1620 began to use the term "Middle Ages," because they regarded themselves as mordern, is an interesting question.

OID comes from the Greek word for "appearance" or "form." Since aster in ancient Greek meant "star," the small bodies oribiting between Mars and Jupiter that looked like stars through primitive telescopes were called asteroids. A factoid is a little bit of information that looks like a fact, whether it is or not. And some people these days will attach -oid to just about anything; you can probably figure out the meaning of nutsoid, nerdoid, and freakazoid without much help.

SCOP which usually appears in a suffix, comes from the Greek skopein, meaning "to look at." In English we have the simple noun scope, along with some other words it sometimes stands for: telescope, microscope, periscope, and so on. And have you ever used a stereoscope, a device your greate-grandparents probably enjoyed, which lets you look through a viewer at two slightly different photographs of the same thing, one with each eye, to enjoy the illusion that you're seeing it in three dimensions?

TRANS comes from Latin to indicate movement "through, across, or beyond" something. Translation carries a writer's meaning from one language to another. A television signal is sent or transmitted through the air (or a cable) to your set. When making your way through a city on public transportation, you may have to transfer from one bus or subway to another.

PRO is an important prefix, with a couple of quite different broad meanings. In this section, we'll look at words in which pro- has the basic meaning "for" or "favoring." Everyone knows words like pro-democracy and pro-American, but other pro- words may not be quite so self-explanatory.

PRO in its other broad meaning, means "before, in front of." So, for example, to proceed means "to move out in front"; to progress means to "to move forward"; and somebody prominent stands out, as if he or she were actually standing out in front of the crowd.

RE is a prefix which, like pro- (see PRO), has more than one meaning. In this section, we'll focus on the meaning "again." We use re- words with this meaning every day--redo, recheck, reread, resell, repaint, etc.--and we feel free to make up new ones as needed. But in plenty of other re- words, the meaning isn't so obvious.

RE in its other main sense, means "back" or "backward." Since doing something again means going back to it, the two senses are actually related; still, the meaning of re- in most words is pretty clearly one or the other. So a rebound comes back at you; to recall means to "call back" a memory; and to react is to "act back" at someone else's action.

DERM comes from the Greek derma, meaning "skin." For medical advice on a skin problem such as acne, we may go to a dermatologist, or skin specialist. When we get a shot, it's usually with a hypodermic, a needle that goes "under the skin" (see HYP/HYPO). A pachyderm is a "thick-skinned" animal, which most of us just call an elephant.

ENDO comes from the Greek endon, meaning "within." In English it appears almost always in scientific terms, especially in biology. A nonscientific endo- word is endogamy, meaning marriage within a specific group as required by custom or law--one of the many customs that can be seen everywhere from the most remote tribes to the highest society in wealthy countries.