Peanut butter is primarily classified as a semi-solid state of matter. Unlike liquids that flow freely or solids that maintain a fixed shape, semi-solids possess characteristics of both. They can hold their shape to an extent but will also deform under pressure.
Peanut butter has a thick and creamy consistency, which allows it to be spreadable yet it doesn’t flow like a liquid. This unique texture is due to the presence of fats and proteins that create a blend of both solid particles and liquid oils. When you apply force, like when you spread it on bread, it responds like a solid, but it will still yield and conform to the surface.
In summary, peanut butter is categorized as a semi-solid, balancing between the rigid structure of a solid and the fluidity of a liquid.