A wave is a mass shipment of toys, consisting of specific toys in specific proportions. For example, a (hypothetical) "Transformers Omega Wave 1" might have shipped on January 15th, 2001, and contained five Optimus Omegas, four Megatron Omegas, four Rodimus Omegas, and one Supreme Omega per case. The next wave of "Transformers Omega" may contain some of the toys from wave 1, mixed with new toys, in different quantities (but retaining the same overall number of individually packaged toys, including multiples of the same toy, contained in a shipping case of each wave). Many times the newest toys tend to be shortpacked compared to the older toys in the same wave.
Each new wave means a new selection of toys appearing on toy store shelves across the country (or world), and many Transformers fans closely follow the news about upcoming waves and what toys will be in them.
Oftentimes, in discussions among fans, the term "wave" might refer only to the new toys that ship in a wave, whilst ignoring carry-overs from previous waves. So if the aforementioned "Transformers Omega Wave 1" contained five Optimus Omegas, four Megatron Omegas, four Rodimus Omegas, and one Supreme Omega per case, and the subsequent "Transformers Omega Wave 2" contains five Wheelie Omegas, three Buckethead Omegas, two Megatron Omegas, one Rodimus Omega and one Supreme Omega per case, many fans will call Wheelie Omega and Buckethead Omega "wave 2 toys", whereas the others, despite shipping in at least two subsequent waves, are usually referred to as "wave 1 toys".
Furthermore, in a case where new waves from different size classes ship together, or around the same time, some fans will incorrectly refer to the entire batch as a "wave". So if a theoretical "Transformers Hyperforce Omega Wave 1", a "Transformers Hyperforce Warrior Wave 1" and a "Transformers Hyperforce Tracker Wave 1" ship together, some fans will reverse this and claim that "Wave 1 of Transformers Hyperforce contains Omegas, Warriors and Trackers". Naturally, this construct requires some clunky adjustment once new waves from different size classes don't ship at the same time.