When Einstein used the term "time dilation," he was referring to the phenomenon where time appears to pass at different rates for different observers, depending on their relative motion or the strength of the gravitational field they are in. It is important to note that time dilation is a consequence of the theory of relativity, specifically the theory of special relativity and the theory of general relativity.
In special relativity, Einstein showed that the passage of time is not absolute but depends on the relative motion between observers. According to the theory, as an object approaches the speed of light, time appears to slow down for that object relative to a stationary observer. This effect is known as "time dilation" because time seems to dilate or stretch out.
Similarly, in general relativity, Einstein extended the concept of time dilation to include the effects of gravity. According to general relativity, the presence of a massive object curves the fabric of spacetime, and this curvature affects the passage of time. In regions of stronger gravity, such as near massive stars or black holes, time appears to pass more slowly compared to regions with weaker gravity.
So, when Einstein used the term "time dilation," he was describing the observed phenomenon that time can slow down or speed up depending on the relative motion or gravitational field strength experienced by an observer. It is indeed a reference to the measurement of time by an observer's clock, which can be affected by motion or gravity.