To hyphenate means to connect two or more words with a hyphen, that short horizontal line that looks like this: -. When you hyphenate words, you're joining them to create a single unit that works together to express one idea.

Writers hyphenate words for clarity and precision. You might write about a “well-known author” (with a hyphen) when it comes before a noun, but say “the author is well known” (no hyphen) when it comes after. Numbers from twenty-one through ninety-nine get hyphens. So do fractions like “two-thirds” and compound modifiers like “state-of-the-art technology” or a “ten-year-old student.”

Sometimes hyphens prevent confusion. “Recover” means to get better after being sick, but “re-cover” means to cover something again, like putting a new slipcover on your couch. The hyphen changes the whole meaning.

The rules for when to hyphenate can be tricky, and they've changed over time. Words that once needed hyphens, like “base-ball” or “to-day,” eventually became single words without them. Other times, two separate words become hyphenated before merging into one, as language naturally evolves. Even professional writers sometimes check their dictionaries to see whether a particular phrase needs a hyphen, so don't worry if you find yourself doing the same.