I came across the term "cluster" in the context of particle physics and was wondering what it actually is.
This answer covers the use of the word "cluster" in the context and analyzing data sets from particle physics detectors. Not sure if there are other contexts or not, but I wouldn't be surprised: it's a big field.
There are two things you have to understand for this to make sense.
First, many particle physics detectors are 'segmented' meaning that they consist of many small detector elements each making up a portion of the detector volume. There are a lot of different schemes for how they are arranged, but it doesn't matter for our purposes. As a result the interaction of a single particle with the detector generates a group of hits in multiple segments.
Second, several classes of particle interaction create a large number of detectable particles going in roughly the same direction. These are called 'showers' and 'jets'.
To identify the particles that make up an event and assign them energies and momenta that approximate their true energy and moment when need to group the signals for analysis purposes in a way that approximate the real origin of each hit in the detector.
This processes is usually called 'clustering' and the groups are called 'clusters'.