Halving
A scheduled event in some proof-of-work networks where the new coins paid to miners for each block are cut in half.
Bitcoin's subsidy halves every 210,000 blocks, roughly every four years, stepping the issuance rate down toward zero and toward the fixed cap on total supply; Litecoin and several others follow the same pattern with their own intervals. The event is automatic and written into the consensus rules, so no committee decides it and every node applies it at the same block height. For miners the effect is immediate and mechanical: the same electricity buys half as much subsidy, so the fee portion of block rewards becomes relatively more important and less efficient hardware becomes uneconomic first. Over the long run the design points to a network paid for by fees alone once the subsidy rounds to zero.
In practice
Once Bitcoin's subsidy schedule reaches zero, block producers would be paid only by the transaction fees users attach, which is why fee revenue is studied closely.
The common misunderstanding
A halving changes issuance, not demand; the schedule has been public since launch, so its arrival is not new information to anyone.