Question

historians today have to be careful about the terms they use because

Answer 1

b'coze there are many terms whose meanings change over a litery source

Answer 2

Historians have to be careful about the terms they use because they denoted several things in the early days.E.g. today, the word ‘foreigner’ represents a person who is not from India. But in the medieval period, a foreigner means any outsider who comes to the village i.e. the one who did not belong to that community was a stranger.
Hope it helps :)