To attach a label to something, typically to provide information about it. She labeled all the boxes with their contents before moving.
To assign a category or name to someone or something, often in a way that is overly simplistic or unfair. He was labeled as a troublemaker by his teachers.
To mark a molecule, cell, or other material with a tracer, dye, or other indicator for identification or analysis. The researchers labeled the DNA with a fluorescent probe.