The term 'gradually decreasing' can be summarized by the word 'WANING', which is a six-letter word. WANING is an adjective that describes something that is gradually decreasing in size, intensity, or strength over time. It is often used to describe the WANING of the moon, which is when the illuminated portion of the moon decreases in size until it is no longer visible. WANING can also be used to describe the gradual decrease in popularity of a product or idea.