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The Vega system or Johnson system of (stellar) magnitudes sets the zero point for a magnitude scale, taking the magnitude of the star Vega as zero. This is used for a general (visible light) magnitude, bolometric magnitude, and for magnitudes associate with individual passbands of photometric systems, i.e., a star's B-magnitude would be zero if it matches the B-magnitude of Vega. Though the Vega system's principle is "Vega's magnitude is zero", in fact, absolute brightnesses are generally used that only approximate Vega's current magnitudes; though Vega was chosen as a star with stable magnitudes, all stars experience some variation and current technology senses that of Vega.
Magnitudes require some adopted zero point as a consequence of their being logarithmic: "zero flux" cannot be used since zero has no logarithm (or its logarithm might be referred to as "minus infinity"). A brightness-level must be chosen to be represented by "zero magnitude", part of the definition of the magnitude's scale.
The Vega system is associated with the UBV photometric system and is often considered part of it, or even the same thing, though the definition of passbands and the definition of magnitude zero-points are not dependent upon each other. The term Vega system is also be used more generically as part of the description or means of using other photometric systems, i.e., merely meaning the magnitudes are calibrated so Vega is zero. The current common alternative to the Vega system is the AB system, which makes no pretense of matching any particular star.