This topic is to clarify about related but possibly different terminologies. Based on a number of posts in the forum, including some in response to my posts, I am getting the feeling that there is a difference between the terminologies time dilation and differential aging, as stated below. I am trying to confirm a few things around this based on the understanding I have got: Is Time dilation a coordinate/observer dependent effect which includes the Doppler effect (apparent aging of a distant body based on Doppler shift of light signals) as well as differential aging (actual difference in the rate of clock ticks which is invariant and not coordinate dependent)? Would it be correct to say that differential aging and relative time dilation can be considered to be the same thing? (Meaning, once the 'apparent' Doppler part (i.e. symmetrical part) of time dilation is taken out, what remains is the "differential aging" or "relative time dilation")? Are the terms time dilation and differential aging treated to be so unambiguously different in accepted/standard scientific literature? (Reason I ask this is I seem to find quite a few references to "time dilation" when the actual phenomenon being talked about seems to be actually "differential aging")?