Timestep, time step, time-step: Which variant to use?

I am writing a piece on integration of differential equations. One of the words that I have to use frequently is "timestep" (however it is written), i.e. a step forward in the "simulated" time.

There seem to be three variants:

• "time step"
• "timestep"
• "time-step"

The wikipedia article one Verlet integration in fact uses all three (!) of these variants.

Is there a context in which one is preferred over the other? How do I decide which one to use?

A general remark on hyphens from Longman English Grammar by Aleander

1 There are no precise rules.

2 When short nouns are joined together, they form one word without a hyphen (a teacup). But this may lead to problems of recognition, therefore bus stop, not busstop.

3 Hyphens are often used for verb + particle combinations as in make-up.

4 When a compound is accepted as a single word, i.e. it has an entry in a dictionary, the tendency is to write it as one word (sunbathing).

5 In other cases the use of the hyphen is at the discretion of the writer, writing paper or writing-paper.

6 But the tendency is to avoid hyphens where possible.

As I was writing this reply, the spellchecker flags timestep as not a word, so based on that I'd avoid it. Whether you need the hyphen or not depends on your usage. If you're using it as a compound adjective, it needs a hyphen; e.x. time-step methods for solving differential equations. The words time and step describe the method together so you hyphenate. If you're using it to describe a discreet interval of time, it does not; e.x. the first time step uses a forward difference, while subsequent time steps use the central difference method. Google "compound adjectives" for more explanation.