vicinal
Concise Oxford English Dictionary © 2008 Oxford University Press:
vicinal /ˈvɪsɪn(ə)l, vɪˈsʌɪn(ə)l/
▶adjective
- 1 neighbouring; adjacent.
- 2 Chemistry relating to or denoting substituents attached to adjacent atoms in a ring or chain.
– origin C17: from Fr., or from L. vicinalis, from vicinus ‘neighbour’.
'vicinal' also found in these Oxford entries:

