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Zero day, in IT, refers to the first day that something is known or anticipated. This term is applied in various ways: for example, the first day that a team of security workers or other party discovers a virus, it is called a "zero day" virus. In other words, zero day is that first day that someone identifies a problem and tries to address a security threat or other IT issue.
In addition to a "zero day virus," IT professionals may talk about a "zero day attack" or a "zero day threat." Zero day is used as a benchmark: often, security teams continue to keep careful track of the number of days that a security issue has been addressed. This is usually done in order to track progress, until a security issue or other IT issue is resolved or closed.
Another role of the term "zero day" is to describe the processes that security workers encounter. Someone might talk about how many "zero day" attacks or threats they found on a given day. Another example is of specific "zero day" vulnerabilities found in the workings of an operating system. As a core software resource, the operating system has a lot of context, and that means that zero day issues carry a lot of this context also. The kinds of new security problems that pros find can be politicized, and affect an OS brand while generating the same kinds of controversy and collaborative fixes that other zero day finds typically involve.