[ T ] to strongly control something:

[ T ] to make something less easily available:

[ I/T ] to make something tighter, firmer, or less easy to move:

also tighten up GOVERNMENT, LAW to make a rule, system, or law stronger and more difficult to ignore:

ECONOMICS, FINANCE limiting the amount of money that people and companies are able to borrow or spend, for example, by increasing interest rates:

ECONOMICS, COMMERCE if a market tightens, there becomes less of something available than is wanted or needed: