Timeseries or Time series? [closed]

This might be a simple one, but I'd like to have your opinion/authoritative statements: I write a thesis in a technical modelling context, where a mathematical model operates on a discrete set of timesteps (or time steps?). Paramaters/Variables that are defined over this set are then called timeseries (or time series?).

A Google battle yields the one-word expressions as clear victors, but dict.leo.org does only like the two-word expressions. So which one is to be preferred? Or is there even a difference in meaning between both variants?

Conclusion: after a temporary confusion from a third option "time-step", I settled for the two word versions for both words.

Solution 1:

In scientific literature, timestep is very common. I am surprised that timeseries beats time series in a google battle though - I would stick with the latter personally.

Solution 2:

I don't see a good reason to join the two into a single word :) Except perhaps when you want to use a "timestep" or "timeserie" as a unit.