A rollover used to be only a serious highway accident, but in the computer world this spelling has also been used to label a feature on a Web page which reacts in some way when you roll the ball inside a mouse or a trackball over it without having to click. It also became an adjective, as in “rollover feature.” However, when giving users instructions, the correct verb form is “roll over”—two words: “roll over the photo of our dog to see his name pop up.”

Since most people now use either optical mice or trackpads the term “rollover” has become technically obsolete, but it persists.

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