Film speed refers to the sensitivity of photographic film to light. It measures how effectively film can capture an image in low-light conditions. There are various terms that can be used to describe film speed such as ISO, ASA, and DIN. ISO is the most widely used system and measures the sensitivity of the film to light. ASA is an older system used primarily in the USA while DIN is a German system. Other terms that can be used are "film sensitivity" or simply "sensitivity". Understanding the terminology can help you choose the right film speed for your photography needs.