Technically a monopoly means that a singular entity has or produces all of something. So to meet the definition of a monopoly they would have to have 100% market share.
Google was declared a monopoly in practice because they were using their market dominance to negatively impact other companies despite not having 100% market share like Microsoft and a bunch of other companies before them. That was about being so dominant and abusing it that action will be taken to keep them from becoming a literal monopoly.