For the prospective college class of 2015, the next three weeks loom large. High-school juniors across the country, facing their first Preliminary SAT exams, are engrossed in improving their vocabulary. Here’s a thought that might help: A word that means the opposite of another is an antonym; a word that looks as if it means one thing but means quite another could be called a phantonym, and warrants wariness.

Phantonyms pop up in the usage of even so careful a speaker as President Obama. As William Safire noted in March, when the president said that he wanted the American people to have “a fulsome accounting” for his stimulus program, he meant full, whereas to punctilious authorities the word means disgusting, excessive, insincere.

Likewise, noisome does not mean noisy but smelly, unhealthful. In a Times book review two years ago, Jack Shafer tartly described a Washington columnist’s “noisome journalistic methods.” Enormity does not mean enormous but great wickedness, a monstrous act. That’s just how Craig Whitney, who will retire this week as The Times’s standards editor, used the term in reviewing a World War II book in May 2008: “The author . . . misses the enormity of what the postwar terrorists did.”

When such terms are misused frequently, some authorities have come to tolerate them as “loose usage.” You need not be a hypercorrective schoolmarm to lament such tolerance. A simple concern for clarity should lead students of all ages to recognize and avoid phantonyms, lest they look unlettered — and lose SAT points. Here are other ghostly misuses that cloud clear language.