Mysterious word, On", appears above posts

Hi,I’m pulling together my first wordpress.org site.
As the topic title says, I have this mysterious “On” that appears on pages and above posts. I can’t locate it in the template html/editor and am hoping someone might be able to tell me where it has come from!
Edit – I’ve done some work on the site since and can’t link you to it just now. But perhaps someone knows the answer without seeing it.
Cheers!