These are related terms, but there's not much consensus about how related; you get different answers from different contexts. (Here's an example.)
There doesn't seem to be much interest in strictly defining these terms on Cross Validated (and I don't think we could even if we wanted to) -- but how do we want to categorize these questions?
- Should we have tags for panel-data and longitudinal-data?
- Should we just stick with time-series?
- Should we make some or all of these into tag synonyms?
(For what it's worth, I tend to think of "time series" as being specific to 2D cases where the dependent variable is the date or time an observation was made and use "panel" and "longitudinal" interchangeably to refer to the more general case with many dimensions.)