"he gave his pet a collar with a metal name tag"

• an electronic device that can be attached to someone or something for monitoring purposes, e.g. to track offenders under house arrest or to deter shoplifters
• a nickname or description popularly given to someone or something

  "he lived up to his tag as the team's saviour"

• a nickname or other identifying mark written as the signature of a graffiti artist

  "scrawled felt-tip tags on city walls"

• an instruction appended to a piece of text in a markup language in order to specify how it is displayed or interpreted
• a word, phrase, or name used to identify digital content such as blog and social media posts as belonging to a particular category or concerning a particular person or topic

  "you can easily add tags to photos en masse"

• the licence plate of a motor vehicle

  "the car had Texas tags"

• a ragged lock of wool on a sheep
• the tip of an animal's tail when it is distinctively coloured
• a loose or spare end of something; a leftover

  "I have a few tags of second-hand equipment"

• a metal or plastic point at the end of a shoelace that stiffens it, making it easier to insert through an eyelet

"his writing is full of tags from the Bible and Shakespeare"

• (in drama) a closing speech addressed to the audience
• a refrain or musical phrase in a song or piece of music
• a short phrase or clause added to an already complete sentence, as in I like it, I do

"mothers suspected that their babies had been wrongly tagged during an alarm at the hospital"

• attach a monitoring tag to

  "we have to tag the sheep before sale or when moving them from one farm to another"

• give a specified name or description to

  "he left because he didn't want to be tagged as a soap star"

• (of a graffiti artist) write one's nickname or mark on (a surface)

  "metal hoardings tagged with hip-hop graffiti"

• add an instruction to (a piece of text in a markup language) in order to specify how it is displayed or interpreted
• add a word, phrase, or name to (digital content) to identify it as belonging to a particular category or concerning a particular person or topic

  "I will be tagged in every photo I post"

• label (something) with a radioactive isotope, fluorescent dye, or other marker

  "pieces of DNA tagged with radioactive particles"

"she meant to tag her question on at the end of her remarks"

• follow or accompany someone, especially without invitation

  "that'll teach you not to tag along where you're not wanted"

• follow closely

  "we were tagged—that car was following us"