In modern English, the term "moment" refers to an unspecific short amount of time. The term is thought to date to the 14th century and was used to refer to a period of 90 seconds. An hour in medieval times therefore consisted of 40 moments. The Hebrew calendar has a shorter definition of a moment, referred to as rega, which is roughly equivalent to 5/144 of a second. In 1990, scientist and poet Miroslav Holub proposed the idea that a moment is about the average length of time it takes a person to read a line of verse.
More about time measurements:
A moment can also be thought of as a reference establishing a single point of time.
"A moment in time" a past occurrence with an unspecified amount of time.