Which word describes a warning often given to someone?

The word that best describes a warning often given to someone is "admonition." An admonition serves as both a caution and a form of advice intended to correct or guide behavior, often implying that there may be consequences if the advice is not heeded. It carries a connotation of seriousness or authority, indicating that the information being shared is important for the recipient's awareness and consideration.

The other options differ in meaning: an adage is a traditional saying that expresses a common truth or observation, rather than a warning. An indication is a sign or piece of information that suggests something may happen, but it is not inherently a warning. Instructions are more directive, focusing on telling someone how to do something rather than cautioning them against certain actions. Hence, "admonition" is the most fitting term for a warning.