What exactly constitutes a monopoly?

Factual Questions
1

For the sake of this discussion we can assume all companies are publicly traded. Or that every company is willing to accept outside investors.

Let’s say i own company A.

I save up a bunch of money and buy a 1% stake in every company in my sector. Is that a monopoly?

Do i need to have a majority share?

2

No, that would not be a monopoly, since your 1% share doesn’t afford you any control over the other companies.

In order to have a monopoly you need to have one entity with effective price control over the entire sector.