What’s the terminology in linguistics for the evolution of “space ship” to “spaceship”?

Question

Is there a term in linguistics for the evolution of a word like “space ship” to “spaceship”? There’s an answer on this site which says:

Here is a rule I read related to evolution of compound words.

Most compound words start out as two words: Air plane, Space ship
Stage two, they are hyphenated: Air-plane, Space-ship
Stage three, over time they start become accepted as a single word: Airplane, Spaceship

I was wondering if there’s a term for this evolution process, to know what term to search for to learn a bit more about it. I vaguely remember reading something like the above before, and had assumed it works that way “as a rule”. But it’s interesting to see the rule doesn’t really seem to apply for the “spaceship” example to begin with: the Google Ngram looks more as if usage of both “space ship” and “spaceship” initially increased in tandem, rather than one preceding the other. A Google Ngram for “spacecraft”, on the other hand, seems to show it didn’t really go through a stage of being “space craft”. It would be interesting to learn more about why different words (especially words as similar as “spaceship” and “spacecraft”) evolve differently in this sense.

Answer 1

The diachronic (what means historical in linguistic) process of forming a new single word from a fixed expression of several words is called univerbation.

In more pedantic words, a linguist may define "univerbation" as a word creation strategy where a complex lexeme once coined tends to become a single complete lexical unit, i.e. a simple lexeme”.

For example, complex prepositions like into, onto, throughout, whereafter, therefore, notwithstanding, hereby arose through the univerbation of frequently adjacent forms.

("English", by Ingo Plag, preprint version, in Word Formation: an International Handbook of the Languages of Europe, edited by Peter O. Mueller, Ingeborg Ohnheiser, Susan Olsen, and Franz Rainer)

A non‐morphological source of complex words is the univerbation (“becoming a word”) of phrases. Phrases may lexicalize into words, and thus lead to complex words. Examples from English are jack‐in‐the‐box, forget‐me‐not (nouns), and dyed‐in‐the‐wool, down‐at‐heel, over‐the‐top.

("Morphology: basic notions", by Geert Booij)