A catch phrase is a phrase or expression that becomes spontaneously popularized (i.e., it "catches" on) through widespread repeated usage, also known as a memetic phrase. Catch phrases often originate in popular culture (such as movies and television), and are spread through a variety of media, including word of mouth. A catch phrase's defining features are its sudden, spontaneous, and widespread public reception, and its adopted use by the public, often to its amusement.

A sign that such a phrase has caught on is its use in many sectors of society — political, social, and cultural. Catch phrases often decline in popularity after a time, but not always. The term "Manifest Destiny", for example, was a catch phrase of the mid-nineteenth century, coined by journalist John O'Sullivan in an editorial in 1845. The phrase spread so quickly that people soon forgot who first introduced the term. In time, "Manifest Destiny" ceased to be a catch phrase, instead becoming a standard historical term, and a permanent part of the lexicon of U.S. history.

Some catch phrases only catch on within specific sub-cultures. In recent times, many such catch phrases are spread through the Internet. The catch phrase "All Your Base Are Belong To Us" is one such example.

Some catch phrases originate as a slogan in an advertising campaign. The catch phrase "Where's the beef?" became popular in the U.S. as a result of 1984 commercial for Wendy's, a hamburger restaurant chain. Advertising slogans that do not catch on with the public do not, by definition, become catch phrases.

Some catch phrases become the "trademark" or defining characteristic of the person or character with whom they originated. A notable example is the catch phrase (actually catch word) "D'oh", the trademark exclamation of Homer Simpson from the long-running animated series The Simpsons, which has now been added to some dictionaries. Another famous catch phrase is "Eh, what's up, Doc?" by Looney Tunes and Merrie Melodies' Bugs Bunny. Bugs usually says this while munching on a carrot.

In professional wrestling, catch phrases are often essential to a wrestler's gimmick. Some, such as The Rock's "If you smell what The Rock is cookin" and Stone Cold Steve Austin's "Austin 3:16" and "That's the bottom line, cause Stone Cold said so" achieved such popularity that they have become symbols of pro wrestling as a whole, even after the active careers of the wrestlers in question had mostly ended.

Pronunciation is essential to some catch phrases. For example, Marv Albert's "Yes!" is a catch phrase mainly due to his distinctive sounding of that word, and people who imitate or parody it try to duplicate his style. Another major example is Curly Howard's (of the Three Stooges) use "certainly" which was distinctively pronounced as "SOYT-enly!"