Answer:
Any small molecule that can bind specifically to its receptor present on or in the cell.
Explanation:
Ligand is a small molecule that can bind a protein molecule called receptor specifically. This binding of ligand brings changes in the cell because it alters the configuration of receptor protein which triggers a reaction sequence in the cell that results in a specific cellular response.
Ligans play a role in cell signaling, cell communication, regulation of gene expression, cell growth and proliferation, etc. So ligands are informatory molecules that give a signal to the cell to perform a specific function. Some of the common ligands are ammonia, carbon monoxide, water, cyanide, chloride, etc.
