causa

Causa, causae is a first declension feminine noun, entirely regular, and it means a cause, a reason, a pretext, or a case in court. Causam agere is to plead a case; causam dicere is to speak in your own defence.

The form that earns the word its rank is the ablative causa used as a postposition. It follows a GENITIVE and means "for the sake of": honoris causa, exempli causa, mea causa, rei publicae causa. Note the word order, because it is the reverse of what Latin normally does with prepositions and it is a reliable marker: if you meet a bare genitive with causa sitting behind it, that is what is happening. Gratia does the same job in the same position, which is where exempli gratia and its abbreviation come from.

It gives cause, accuse, excuse and because. Its most surprising descendant is French chose, Italian cosa and Spanish cosa, all meaning simply "thing": the Romance languages picked the legal "matter" up and wore it down into the vaguest noun they have.