The term "abyss" has a number of meanings, depending on its context. In general usage, it refers to an deep and immeasurable depth or profundity; something that is very difficult to understand or fathom. It can also refer to the bottomless pit or hell in some religious beliefs.
In geology, "abyss" specifically describes a deep sea trench or depression in the ocean floor. In astronomy, it refers to a hypothetical region of space beyond our observable universe, often used as a metaphor for something unknown and mysterious.