I thinked "raid" was when many monsters spawns some times and "invasion" was when a boss comes to Tibia, but is not true.
Looking for a definition on fansites:
I found in tibia.fandom.com a page about raids, here https://tibia.fandom.com/wiki/Raid. In this page we can find Grynch Clan Goblin Invasion and Undead Jester Invasion. So we have raids and invasions used for the same event.
Looking for a definition on Tibia:
In the isle of the kings library has a book "Demonic Invasions Paragraph I (Book)" that use the word "Invasion" to define when a boss comes to Tibia, but the tibia client has a notify that says "Active raid".
So, in practice, we have not a difference.