"3-" refers to a specific type of ion known as a triiodide ion, which consists of three iodine atoms bonded together. It carries a negative charge, making it an anion. This ion is often encountered in various chemical reactions and is important in the field of chemistry, particularly in iodine-based compounds. In solution, the triiodide ion can form when iodine dissolves in a solvent, such as water or alcohol, often in the presence of iodide ions. It is commonly used in laboratory settings and has applications in medical imaging and antiseptics.