Substance
Formal Statement
By substance, I mean that which is in itself and is conceived through itself — that is, that whose concept can be formed without needing the concept of any other thing. Substance is ontologically and conceptually independent.
In Plain Language
Imagine something that does not lean on anything else for its existence or its intelligibility. You do not need to refer to anything outside it to understand what it is. That is what Spinoza means by "substance." It is the most self-standing thing there could be — the bedrock category of his entire system.
Why This Follows
This is our starting point — the first definition from which the entire system will be built.
Substance is whatever exists in itself and is conceived through itself.
Connected Concepts
Can you think of anything in everyday experience that truly requires nothing else to exist or to be understood?