What are terrorism threat levels?

Terrorism threat levels indicate the likelihood of a terrorist attack in the UK. There are two published threat levels. 

There are 5 categories at which the threat levels could be set: 

LOW - an attack is highly unlikely 

MODERATE - an attack is possible, but not likely 

SUBSTANTIAL - an attack is likely 

SEVERE - an attack is highly likely 

CRITICAL - an attack is highly likely in the near future