I'd say a "reorg attack" is a type of 51% attack (perhaps a big subset of attacks that could occur once someone controls more than 50% of hashing power).

There's a chance not all 51% attacks are reorg ones though as some may try to change other things (such as enforcing a malicious consensus on the protocol - for example to credit themselves a certain amount of btc). It might also be argued it becomes a denial of service attack once the reorg is broadcast while the hashing power majority remains with the attacker.