Can anyone explain to me what is meant by a 'pure' time series model?
I believe it might have something to do with the exclusion of external factors but I'm really not too sure.
For example, the term is used here[1] (Sec 4.8),
a pure time series based approach can be used to generate forecasts for the predictor variable
And also in a post here on Cross Validated:
consider a pure time series model with AR(1) DGP: $x_t=ϕx_{t−1}+c$
[1]: Hyndman, Rob J and George Athanasopoulos,
Forecasting: principles and practice
https://www.otexts.org/fpp