is hacking illegal?

It all depends on what you are hacking. Stealing your brothers jeans to wear on a night out isnt really illegal, but its still taking something without permission. It all comes down to context. Hacking in itself is not a word that implies someone is doing something illegal. It means they are trying to perform an action that the original designer of a piece of software had not originally intended. Sometimes hacks can be good things sometimes they are bad. 

Hacking a bank to rob money is completely illegal, but hacking a tool to make it perform better is perfectly fine. Software developers will hack their own code all of the time. Sometimes ""hacking"" is used very loosely. If you have someones password and username and you log into their account to steal money. People call it hacking, but it isn't a hack at all. 

So to go back to the original question on the legality of hacking. No, hacking is not illegal. Stealing things online by breaking into systems you shouldn't have access to is illegal. It is just so common because it is hard to catch the people who do this stuff in the first place.