P1

p1 is a compact designation used in multiple disciplines to label the first item in a sequence, a parameter, or a code. It does not have a single, universal meaning; its interpretation depends on the domain and the surrounding notation.

In mathematics and computer science, p1 often appears as a subscript or identifier for the first element of a list or sequence, such as p1 representing the initial parameter p_i with i = 1, or as a variable name in equations and algorithms.

In information technology and operations management, P1 (capital P, number 1) commonly denotes the highest priority category, used for critical incidents or tasks that require immediate attention. Service-level agreements and issue trackers frequently define P1 as the most urgent level.

In biology, P1 can refer to the P1 antigen, part of the P blood group system. The P1 antigen is a carbohydrate structure expressed on the surface of red blood cells; variations in expression give rise to P1 and related phenotypes.

Other uses include product and hardware naming, where p1 may mark a first-generation model or a variant in a family of devices or software. Because meanings are context-dependent, consulting domain-specific documentation is recommended.