A researcher was an individual that conducted research.
Professor Donald Kelt was the lead researcher at the Lashley Institute. (PROSE: Artificial Intelligence)
Mark Gregory was a researcher for International Electromatics. He assisted in developing the Cerebration Mentor for use as a weapon against the Cybermen. After UNIT rescued Professor Watkins, the Cybermen shot and killed him. (TV: The Invasion)
Julian St. Stephen was a researcher who worked as a consultant to the Ministry of Defence in 1964. (AUDIO: Artificial Intelligence)