The Institute of Electrical and Electronic Engineers (IEEE) is the organization responsible for the standards that define modern wireless enterprise networks, for example, WLANs. Industry interests are coordinated through the Wi-Fi Alliance. They select the subset of features that become the minimum set of features for certified hardware, software, and devices. They also control the use of trademarks.
The IEEE uses a unique numbering sequence for every standard. The 802 standard is for networking but is further broken down into 22 subparts. 802.11 is the sub-part for wireless networking. Over the past 20 years, the 802.11 standards has been updated many times beginning with revision 802.11a and continuing through 802.11ax. Some of the updates addressed specific technical shortcomings (for example, QoS) of the original standard, while others substantially changed fundamentals of the wireless protocol (for example, QAM modulation). In retrospect, the fundamental changes were generational changes.
It is simply the IEEE and Wi-Fi Alliance acknowledging the benefit of explicitly identifying the major generational changes within the 802.11 specification family. See the table below.