A re-agent is a substance that is added to a chemical or biological system to cause a reaction or to test for the presence or absence of certain substances. There are many synonyms for the word re-agent, including reactant, chemical agent, reagent substance, test agent, and assay agent. Each of these words can be used interchangeably with re-agent depending on the context and specific application. Re-agents are essential in scientific research, diagnostics, and quality control in a variety of industries. By having a variety of synonyms for the word, scientists and researchers can convey their ideas and results more precisely and effectively.