paste (rheology) paste (rheology) in physics, a paste is a substance that behaves as a solid until a sufficiently large load or stress is applied, at which point it flows like a fluid. in rheological terms, a paste is an example of a bingham plastic fluid. pastes typically consist of a suspension of granular material in a background fluid. the individual grains are jammed together like sand on a beach, forming a disordered, glassy or amorphous structure, and giving pastes their solid-like character. it is this "jamming together" that gives pastes some of their most unusual properties; this causes paste to demonstrate properties of fragile matter. examples include starch pastes, toothpaste, mustard, and putty. in pharmacology, paste is a basic pharmaceutical form. it consists of a fatty base (e.g., petroleum jelly) and at least 25% of a solid substance (e.g., zinc oxide). pharmaceutical pastes are typically intended for external application to the skin. they are usually thick and do not melt at physiologic temperatures. properties of paste