Wiktionary
clavam
n. (context chemistry English) A molecule similar to a penam, but with an oxygen atom substituted for sulfur, such as clavulanic acid.
Wikipedia
Clavam
A clavam is a molecule similar to a penam, but with an oxygen substituted for the sulfur. Thus, they are also known as oxapenams.
An example is clavulanic acid, from which this compound class receives its name.