Final answer:
A signal molecule is known as a ligand, which binds to a receptor protein on a target cell, initiating a signaling pathway within the cell.
Step-by-step explanation:
A signal molecule is also known as a ligand. Ligands are molecules that bind to another specific molecule, often a receptor protein in or on a target cell, triggering a cascade of events within the cell known as a signaling pathway or cascade. This process can result in changes within the cell, such as alterations in gene expression or metabolic activity. Ligands are a crucial part of cell signaling, as they initiate the communication between cells that is necessary for various biological processes.