When you look at a piece of code/software, it is often fairly intuitive to know where/what are some of the implemented features; either because there is a good amount of documentation, the code is well structured, or because there are tools that help with feature location. However, not all features are simple to find: some because of poor maintenance, others because the feature was not necessarily treated as such during the implementation. I would call these ad-hoc features. As the name suggests, these features were implemented without much thought: maybe developers thought it would not be useful to properly package them and they ended up being reused elsewhere, maybe a large team got confused and re-implemented the same piece of code many times, or maybe someone just didn't recognize that there was even a feature to begin with!