usputan

1. (of a remark, conversation) casual; passing; incidental
   To je bila samo usputna primedba, ne ljuti se. — It was just a passing remark, don't be upset.
   Upoznali smo se u jednom usputnom razgovoru. — We met in a casual conversation.
2. (of a place) along the way; intermediate
   Autobus staje na svakoj usputnoj stanici. — The bus stops at every stop along the way.

The related adverb is usput 'on the way; by the way'.