What is a "negative function" withing algorithms?
The big-O notation (and also similar notations, such as big-Omega) are not inherently limited only to describe running time of algorithms. Indeed, when the context is the running time of some algorithm, there are no negative functions since algorithms cannot run &quot;negative time&quot;.
That being said, the big-O notation is a general mathematical definition. It could be used for other things as well, as shown in this Wikipedia page.