refers to the unique DNA recombination event that took place in one plant cell, which was then used to generate entire transgenic plants
Every cell that successfully incorporates the gene of interest represents a unique "event". Marker genes are used to identify transformed cells, and each resulting transgenic plant is the result of one event. The derived transgenic line is identified by an abbreviation (e.g. Bt11, MON 863).
Although many different transgenic plants are made with the same gene construct, only a handful merit further use. Different events can have much different consequences. This depends on the number of times the gene construct was added to the cell's genome and may also have something to do with the placement of the new genes. The events that result in optimal transgene expression and traits are considered "elite" events.
Every plant line derived from a transgenic event is considered a GMO. Its commercial use and release into the environemnt require authorisation according to EU regulations. Events can be introduced to other cultivars by breeding. This is why certain events (e.g. MON 810) are available in many different cultivars.