What is the definition of "gasLimit" in the chain spec files for Parity? The wiki says it is "gas limit of the genesis" ! If I knew what that definition meant, I wouldn't need a definition.
Each ethereum transaction uses "gas" to protect miners from running infinite computations. Transaction senders may send a gas limit which indicates how much gas they are willing to spend on processing a particular transaction (in general it is impossible to know in advance how much gas a transaction will require).
A block also has an overall gas limit, to prevent blocks from getting too big, and processing times from getting too high. The genesis block is no different in this regard - the setting you refer to is the gas limit for the genesis block, i.e. how much gas may be used when mining all transactions in the genesis block.
gasLimit parameter in Parity chain spec file refers to the Block Gas Limit. It is a default value that may only apply to the genesis block, as miners can "vote" to raise or lower the Block Gas Limit over time.