Subletting
What it really means
Renting out your room or flat to someone else while you are the tenant, for example over the summer. Most contracts ban it without the landlord's written permission, and breaking that rule can end your tenancy. Always ask first.
Context, tips & common mistakes
Almost every tenancy agreement bans subletting without written consent, and breaching it is one of the fastest routes to eviction proceedings and a burned reference — this includes 'just a friend paying a bit while I'm away', and very much includes putting the flat or your room on Airbnb, which agents actively scan for. Whole-property sublets can also void the landlord's insurance and mortgage conditions, which is why consent is rarely casual.
If your circumstances change mid-tenancy — leaving the country, course ending early — the legitimate routes are better than the covert one: ask the landlord about assigning the tenancy or a replacement tenant (on a joint student let, a replacement housemate is routine and usually welcomed), negotiate an early surrender, or check for a break clause. Taking in a lodger while you remain living there is a different arrangement but still needs the same written permission first.