sesqui-
Concise Oxford English Dictionary © 2008 Oxford University Press:
sesqui-/ˈsɛskwi/
▶combining form denoting one and a half: sesquicentenary.
■ Chemistry (of a compound) in which a particular element or group is present in a ratio of 3:2 compared with another.
'sesqui-' also found in these Oxford entries:

