Feature creep (sometimes known as requirements creep or scope creep) is a tendency for product or project requirements to increase during development beyond those originally foreseen. Feature creep may be driven by a client's growing "wish list" or by developers themselves as they see opportunity for improving the product. To control feature creep, project management tools, such as the requirements stability index (RSI), are sometimes advocated.
This PM training course video provides three tips for preventing feature creep. (You can skip the ad after five seconds.)