What does the word mixture mean to a scientist?
It means the same thing as it does to anyone else: a combination of two or more different components.
They might be all gases, or all liquids, or all solids, or some gases and some liquids, or some liquids and some solids. (Almost never do we talk about a gas and a solid mixing though.)
What matters is that, well, it's matter, of a variable composition.
Typically, chemists differentiate between homogeneous and heterogeneous mixtures. All that means is that the mixture is either uniform or nonuniform.
That is, you can either not pull things out by hand (homogeneous), or you can (heterogeneous), i.e. the mixtures are heterogenous if they are separable by physical means, and homogeneous if not.