What is a 'time-variant' signal? I have found in the literature a "time-variant signal" term , but I could not understand what that's mean. I know that there is "time-variant systems", but this is the first time, which I see a time-variant signal term. Any one has an idea what this term means ?   Some suggests that this term is related to non-stationary signal, which it's spectrum changes over time. Is this right?

P.S: I have added the  EEG tag because this term, is used widely to describe the EEG signals
As I've mentioned in a comment, time variant signal is a term which is not commonly used in signal processing, and I also couldn't find a clear definition. After browsing a few texts which use the term (usually written by people not specialized in signal processing), I've come to the conclusion that by time variant signals they actually mean non-stationary signals. The latter being a clearly defined term (check the wikipedia page on stationarity). In short, it means that the signal's properties, such as its mean and variance, change over time. And this is indeed the case with EEG signals, just as it is the case with speech signals.