Oscillator
Definition 1: A form of momentum or rate-of-change indicator which is usually valued from +1 to -1 or from 0% to 100%.
Source: Edwards, Robert and Magee, John.Technical Analysis of Stock Trends 9th Edition; (c) 2007.
Definition 2: Oscillators are indicators that are designed to determine whether a market is “overbought” or “oversold.” Usually, an oscillator will be plotted at the bottom of a graph, below the price action. As the name implies, an oscillator is an indicator that goes back and forth within a range. “Overbought” and “oversold” conditions (the market extremes) are indicated by the extreme values of the oscillator. In other words, as the market moves from overbought, to fairly valued, to oversold, the indicators have different ranges in which they vary. Often, the oscillator will be scaled to range from 100 to -100 or 1 to -1, but it can also be open-ended.
Source: Kirkpatrick, Charles and Dahlquist, Julie. Technical Analysis: The Complete resource for Financial Market Technicians; (c) 2007.