pars pro toto
Concise Oxford English Dictionary © 2008 Oxford University Press:
pars pro toto /ˌpɑːz prəʊ ˈtəʊtəʊ/
▶noun formal a part taken as representative of the whole.
– origin L., lit. ‘part on behalf of the whole’.
'pars pro toto' also found in these Oxford entries:

