In musical terms, which word signifies a very slow tempo?

The term that signifies a very slow tempo is "lento." In the context of musical terminology, "lento" specifically describes a tempo that is slow and leisurely, often associated with a flowing, calm character in the music.

Largo, also an indication for a slow tempo, is often considered even slower than lento, but it is used less frequently and can denote a broader spectrum of slow rather than the precise calmness that "lento" conveys.

Marcato, on the other hand, refers to a style of playing that is accented and marked, rather than indicating a specific tempo. Leggiero describes a light and delicate touch, typically informing the performer to articulate notes in a swift and agile manner, which contrasts with the slow nature of "lento."

Thus, "lento" is the most accurate term for describing a very slow tempo in music, making it the correct answer.