In linguistics, univerbation is the diachronic process of combining a fixed expression of several words into a new single word. Some English examples include always from all [the] way (the s was added later), onto from on to, albeit from all be it, and colloquial gonna from going to.
The process is epitomized in Talmy Givón's aphorism that "today's morphology is yesterday's syntax".
References
Univerbation Wikipedia(Text) CC BY-SA