"Monopoly" is not a great word anyway because it does not really capture what the law of almost every modern free market economy forbids. "Anti-competitive" is more accurate.
Using that definition it becomes easier to see how a company can have a large majority of the market share and not be a monopoly, because it is not engaging in anti-competitive behavior, and conversely, how a company can have a non-majority share of a market but still be considered anti-competitive (by for instance price fixing, or vertical market manipulation, or collusion with competitors, etc.).
Using that definition it becomes easier to see how a company can have a large majority of the market share and not be a monopoly, because it is not engaging in anti-competitive behavior, and conversely, how a company can have a non-majority share of a market but still be considered anti-competitive (by for instance price fixing, or vertical market manipulation, or collusion with competitors, etc.).