tighten in WordNet English dictionary

1. restrict
   "Tighten the rules"; "stiffen the regulations"
   stiffen  tighten  tighten up  constrain 
   
2. make tight or tighter
   "Tighten the wire"
   tighten  fasten 
   
3. narrow or limit
   "reduce the influx of foreigners"
   reduce  tighten 
   
4. become tight or tighter
   "The rope tightened"
   tighten