Chemistry - Is toothpaste solid or liquid?
Toothpaste is what is called a non-newtonian fluid, more specifically toothpaste is a Bingham plastic. This means that the viscosity of the fluid is linearly dependent on the shear stress, but with an offset called the yield stress (see figure below). This yield stress is what makes it hard to say whether it is liquid or solid. The fact that toothpaste is viscous alone is not sufficient to explain this, because water is also viscous, but doesn't behave like a solid (unless frozen, but that's another phenomenon).
What the yield stress does is the following. Below a certain shear threshold the fluid responds as if it were a solid, as you can see happening when you have put toothpaste on your toothbrush, it just sits there without flowing away. A highly viscous but newtonian fluid would flow away (although slowly as pointed out by @ron in his comment to the answer of @freddy).
Now if you put sufficient shear stress on the toothpaste, when you squeeze the tube of paste, it will start flowing and respond as a liquid.
Other examples, as mentioned in the Wikipedia link in my first sentence, are e.g. mayonnaise and mustard. Another example is silly putty.
Here's a genchem-level answer for a genchem-level question about the classification of matter:
Toothpaste is a sol: a stable suspension of tiny solid particles in a liquid. When the toothpaste dries out you can see what the solid part alone looks like.
Mixtures with more than one phase often have interesting properties and behaviors that the components alone don't have; the other answers to your question touch on some of these.
Other examples of sols are paints and solid-pigment inks. Again, you can see the solid part when they dry.
Here's the boring answer: Toothpaste is a mixture of some solids and some liquids.
The question "is it solid or liquid?" makes sense when you're talking about a substance or a mixture with a single phase—that is, a substance or mixture that's pretty much completely uniform throughout space. Examples of single-phase materials include pure water (a liquid), a chunk of copper (a solid), and sugar water (which is a liquid—including the sugar).
But some materials consist of multiple phases, and in this case, we have to ask the question "is it solid or liquid?" separately for each phase. Examples of multiple-phase materials include ice water (a mixture containing both a solid (ice) and a liquid (water)), vinaigrette (a mixture of oil (a liquid) and vinegar (another liquid)), and toothpaste (a mixture of several kinds of solids and liquids).
It is Viscous.
Viscous mean "having a thick, sticky consistency between solid and liquid"(dictionary meaning)
There are many more examples like tomato ketchup, honey, wax, toothpaste, etc.
To know more check out Wikipedia
Actually, toothpaste is both. I'm no chemist, but I am pretty sure that it can be correctly classified as a semisolid, which means exactly what you'd think. Semisolids have properties of both solids and liquids. Slime would be another example of a semisolid.