Which term indicates a gradual increase in loudness?

A gradual increase in loudness is referred to as "crescendo." This term originates from the Italian word meaning "growing" and is commonly used in musical notation to instruct performers to gradually increase the volume of their playing or singing.

Crescendos are often notated in music with the symbol that visually represents a widening angle, indicating the need for the performer to subtly build intensity over a specified duration. This technique is important for creating dynamic contrasts within a piece and helps convey the emotional arc of the music.

In contrast, the other terms listed pertain to different concepts. "Diminuendo" signifies a gradual decrease in loudness, while "forte" indicates a loud volume as a static dynamic level. "Staccato" refers to a style of playing or singing where notes are performed in a detached manner. Understanding these distinctions is crucial for interpreting and executing musical directions accurately.