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In computing, the term " token" has several meanings depending on the context, primarily in programming/compiler design and computer security/networking. In general, a token is an object or piece of data that represents something else, often a larger or more complex piece of information, to streamline processing or enhance security.

In Programming and Compiler Design: In programming languages and compiler design, tokens are the smallest meaningful units of source code, serving as the fundamental building blocks. During a process called lexical analysis (or "lexing"), the compiler breaks down the source code into a sequence of tokens. Common categories of programming tokens include:

Keywords: Reserved words with special meaning (e.g., if, while, function).

Identifiers: Names given to variables, functions, or classes by the programmer (e.g., user_name, calculate_total).

Operators: Symbols that perform operations (e.g., +, -, =, *).

Literals (or Constants): Fixed values (e.g., 100, "hello", true).

Separators (or Punctuators): Punctuation marks used for structure (e.g., ;, {}, ,).

In Computer Security and Networking

In cybersecurity, a token is a credential used to verify identity and control access to systems or resources.

Authentication/Access Tokens: After a user logs in with credentials, the system issues a unique, often encrypted, string of characters called an access or authentication token. This token is used for subsequent requests, acting as a "digital key" that proves the user has been verified without requiring them to re-enter their password every time. They typically have a limited lifespan and can be revoked.

Security Tokens (Hardware/Software): This can refer to a physical device (like a USB key, smart card, or key fob) or a software application that generates time-sensitive codes for two-factor authentication (2FA).

Tokenization (Data Security): The process of replacing sensitive data (like a credit card number) with a nonsensitive, unique identifier (the token) to reduce security risks.