Final answer:
The liquid mixed with acrylic powder to create an overlay is a monomer, which acts as a binder and solvent to form a durable hard substance used in applications like nail enhancements.
Step-by-step explanation:
The liquid that is mixed with the acrylic powder to create an overlay is called a monomer. When combined with the acrylic powder, a polymer, it forms a hard substance used in various applications such as nail enhancements. The monomer liquid acts as a solvent and binder that, when mixed with the polymer powder, causes a chemical reaction known as polymerization. This process enables the acrylic product to harden and bond to the surface it is applied to, creating a durable layer. The monomer is crucial in the process as it effectively turns the powdered polymer into a malleable form before hardening.