Your ability to mimic Webster's Dictionary isn't evaluated by most standardized exams, so you won't be expected to provide definitions for difficult words. However, your vocabulary will come in handy for the many indirect and hidden vocabulary questions you'll encounter in tests like the SAT, ACT, PSAT, and others.
Why your test prep should include vocabulary
It's pretty common for tests to have reading comprehension passages that sometimes include vocabulary-in-context questions. These focus on particular words in the passage and ask you to determine their meaning. Sometimes these are "hard" words, but more often they are deceptively "easy" words that actually have several possible meanings. The more extensive your vocabulary is, the better chance you'll stand of answering questions quickly and correctly.
A solid vocabulary can ease the large amount of required reading on most standardized tests (such as the SAT, ACT, and PSAT), as well as the occasional math item that's made a bit more complicated with the inclusion of a challenging word.
So, there's no doubt about. Your vocabulary matters to your test preparation plan, but how do you go about building it?
Create your own word list
Get into the habit of reading every day, with your dictionary and a notebook nearby. When you encounter a new word, look it up. Next, jot it down, along with its definition and the sentence in which you encountered it. Review your list periodically — say, once a week. Making the effort to write down the words and their meanings will help fix them in your memory.
Study vocabulary daily as part of your test preparation
There are some topics for which you can easily cram. Vocabulary isn't one of them. Words generally stick in the mind not the first or second time you learn them but the fourth or fifth time. As part of your test prep, try to start studying vocabulary several weeks before the exam for 15 to 20 minutes a day. Periodically review all the words and quiz yourself, or have a friend quiz you. This simple regimen can help you learn several hundred new words.
However, don't try to gobble dozens of words in one sitting. They're likely to blur into an indistinguishable mass. Instead, pick a reasonable quantity of around 10 to 15 words and study them in-depth.
Learn words in families
Language is a living thing. Words are the product of creative beings who twist, reshape, invent, and recombine them. (Think of the new language being created online, for example.) As a result, most words belong to families, in which related ideas are expressed through related words.
For example, the adjective anachronistic means "out of the proper time". Here's an example of the word in context: The reference, in Shakespeare's Julius Caesar, to "the clock striking twelve" is anachronistic, since there were no striking timepieces in ancient Rome.
When you learn this word, you should also get to know related words. The noun anachronism means something that is out of its proper time. The clock in the above example is an anachronism; so are the pants worn by modern baseball players, which reflect a style that went out of date generations ago. When you learn the adjective, learn the noun (and/or verb) that goes with it at the same time.
Learn word roots
The two words we just discussed are like brother and sister. Their origins can be traced back to the Greek word chronos, which means time. As you explore vocabulary during your test preparation, you'll find that many words come from Latin and Greek. As people in England and America imported words from Greek and Latin, they rarely imported just one from a given root. Thus, a root can help you to learn several words at once. In addition to anachronism and anachronistic, chronos is the source of the words chronic, chronicle, chronograph, chronology, and synchronize. All have to do with the concept of time.
Use the words you learn
Include the new words you're learning through your test prep in your daily speech and writing. It will impress people (teachers, bosses, friends) and it will help solidify your memory of the words and their meanings. Maybe you've heard this about meeting new people: If you use a new acquaintance's name several times, you're likely never to forget it. The same is true with words: Use them, and you won't lose them.