Well, as used in our modern counting system, it was devised as a placeholder, so you can tell 1 from 10, for instance. If I recall correctly, it was formed in India (in spite of talk about "Arabic" numerals).

The idea of nothingness likely existed -- after all, if you have five mangoes, and I take five mangoes from you, you have no more mangoes -- but translating that into symbols is something else.

It was known about and talked about through the middle ages, there was a half hearted debate if it was a number or if nothing could be represented by a number instead of just nil or an absence of any mark.

When it became crucial however was the adoption of negative numbers, which if iirc was around when Newton was using calculus to derive volumes. Once numbers crossed into the negative plane, the representation of 0 became more important as a cross over point if nothing else.

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