Definition 1:

A continuous, measurable quantity in which events occur in a sequence proceeding from the past through the present to the future.
Science Dictionary

Definition 2:

The measured or measurable period during which an action, process or condition exists or continues.
Merriam-Webster Dictionary

Definition 3:

A dimension in which events can be ordered from the past through the present into the future, and also the measure of duration of events and the intervals between them.
Clearly time is not an object or substance we can touch or see. But neither is it merely a dimension, quantity or a concept. Indeed, time has many aspects and appears to represent different things to different people in different circumstances.