According to the vedas, a thief is someone who takes more than he needs.
God has created the world to have enough resources for everyone, but not enogh for everyones greed.
What you really need is defined as what you need to maintain your body, which is very little.
Animals follow that law very closely, they might seem to just "steal" what they need and even kill other life to maintain theirs, but they hardly ever take more than they need, this gives a good understanding what is meant by what you really need.
In the human lifeform however there are exceptions, since a human is capable to more than an animal. As a human you may take more resources than you need for your own survival, if you can make meaningful use of those, for example something that will benefit others as well.
So according to that definition hoarding riches is equally thievery than just stealing things that do not belong to you, maybe the hoarding part is even more thievery, since a lot more of resources are stolen from other living beings.
This is more the definition on the material side, but there is also a spiritual side of thievery. On the spiritual side a thief is defined as someone who steals spiritual knowledge from someone, but this is another topic, since those thieves are often metaphorical and not real people, it is more that the sins of a person make him become a thief, which ties back to why someone becomes a material thief, the sins are all interconnected.