‘Domicile’ is a legal word generally used to define the place where you have your permanent home. It is therefore used more to define your long-term habitation, as opposed to your residency, which is more short-term.
The word ‘domicile’ doesn’t have a set meaning, but is typically used to try and establish the country that you call home. Many people have more than one residence, but the domicile will be the place where they have a residence and the intention to live long term.
Often, your domicile is deemed the same as your father (or your mother if unmarried), but this can be displaced if you settle permanently in another country. You can also acquire a UK domicile if you have been resident in the UK for at least 15 of the last 20 years.