The term "021" can refer to multiple things depending on context, but it is most commonly recognized as a telephone area code in various countries. Some notable uses of 021 as a telephone area code include:

• In Brazil, 021 is the area code for Rio de Janeiro and surrounding cities.
• In China, 021 is the area code for Shanghai.
• In Indonesia, 021 is the area code for Jakarta and its surrounding cities.
• In Iran, 021 is the area code for Tehran.
• In Ireland, 021 is the area code for Cork city and parts of County Cork.
• In Libya, 021 is the area code for Tripoli.
• In Pakistan, 021 is the area code for Karachi.
• In Serbia, 021 is the area code for Novi Sad.
• In New Zealand, 021 is used as an area code for cellular phones on the Vodafone network.
• In South Africa and Sweden, 021 is also used as an area code for certain cities.

Additionally, 021 can sometimes represent negative acknowledgment (NAK) in communication protocols, and it may be a city code or dialing code for specific regions like Cape Town. Thus, the meaning of "021" depends on the context, especially which country or system it relates to.