1. an accumulation of something, growing in size/volume
the build-up of tension/pressure/dirt
This expression refers to a gradual increase in the amount or level of something over a period of time. It most commonly used in a negative sense, and can often be as a result of neglect or disuse. For example, you may notice a 'build-up' of dust on some of the surfaces in your house if they haven't been cleaned in a while. Day-to-day, you may complain about a 'build-up' of traffic on the motorway, or seasonally, a 'build-up' of snow outside. It is therefore mostly used in a social context, and shares a similar meaning to the term 'accumulate.'