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Polyphase AC is a system for generating and transmitting alternating current (AC) electricity using multiple phases. Instead of a single alternating voltage, polyphase systems employ two or more AC voltages that are offset in time, or phase, from each other. The most common polyphase system is three-phase AC, which uses three voltages that are 120 degrees apart. Other systems, such as two-phase, exist but are less prevalent.

The primary advantage of polyphase systems over single-phase AC is their efficiency in power generation and transmission. Generators producing polyphase power are simpler and more robust, with a more constant power output compared to single-phase. This constancy is crucial for heavy machinery. In transmission, polyphase systems require less conductor material for the same amount of power delivered, leading to lower infrastructure costs and reduced energy loss.

Polyphase power is particularly well-suited for driving electric motors. Polyphase induction motors, a common type, are self-starting and offer higher efficiency and power output than their single-phase counterparts. This makes polyphase AC the standard for industrial applications and large commercial buildings where powerful motors are in regular use. Residential power is typically single-phase for simplicity and cost-effectiveness, though some larger appliances might utilize split-phase, a derivative of single-phase. The principles of polyphase AC are fundamental to the modern electrical grid.