Block Subsidy is the newly created part of mining revenue. Coinbase may also allocate fees, which transfer existing bitcoin. Total reward and new issuance therefore differ.
On mainnet Bitcoin Core starts at 50 BTC and halves the satoshi integer every 210,000 heights. At height 840,000 the limit is 3.125 BTC. Halving is neither a separate payment nor a price response; actual block pace determines its date. [Bitcoin Core v29.0: subsidy and coinbase validation]
Nodes sum ordinary input-output differences as fees and compare coinbase with fees plus subsidy. Exceeding that limit invalidates the block despite sufficient Proof of Work. Claiming less is allowed; unclaimed issuance does not carry over to the next block. [Bitcoin Core v29.0: subsidy and coinbase validation]
A coinbase output at H can first be spent at H+100. It must also remain in valid active history; an abandoned block contributes no reward there. Genesis Block has a special unrecorded output, distinct from ordinary maturity. [Bitcoin Core v29.0: subsidy and coinbase validation]
Integer halvings make subsidy zero from height 6,930,000, approximately 2140 at target pace. Fees may continue, but neither their amount nor the security budget is guaranteed. Check height, GetBlockSubsidy and actual coinbase, not only total reward. [Bitcoin Core v29.0: subsidy and coinbase validation]
For the clearest picture, read this entry together with Halving, 21 Million, Mining, Transaction fees, Genesis Block, Coinbase transaction. The reverse links also lead from Genesis Block, Proof of Work, Mining, Difficulty adjustment.