A wave attack is sending 3 fleets (or sets of ships) on attack mode to a planet or moon within 30 minutes of the first attack. This constitutes only 1 attack. Therefore, a player can send 100 battleships and 20 large cargos to attack Planet "Colony", wait 5 minutes and send another 50 battleships and 10 Large cargos to attack the same planet; wait another 5 minutes and send another 25 battleships and 5 large cargos. Since each subsequent attack hit the planet within the 30 minutes of the FIRST attack, this "wave" is considered 1 attack. This can be done 2 more times for an equivalent of 9 attacks, or 3 waves.
The reason for a wave attack is because if a player has a good amount of resources this is the best way to get as much of it as you can without breaking the bashing rules.
Remember: the waves apply PER planet/moon - not player - so you can wave a player's planet AND a player's moon with the same coords (18 attacks; 6 waves; 3 waves against planet and 3 waves against moon). Therefore, it's technically possible that in the same day you can hit the same PLAYER (if he has 8 colonies plus his home planet, and moons on each planet) 162 times in 1 day.
Note: With the new rules, only 8 attacks (fleets) are permitted per day, per planet/moon. [1]
Important note[]
This rules are subject to change by GameForge and may even be different in each community. In some servers, there is no such concept as "wave attacks" so attacking anyone more than 6 or 8 times in 24 hours on the same planet/moon will end up with a bashing ban. To learn the rules for your community, open a ticket on ogame servers or post in the server forum.