Delta modeling is an approach to facilitate automated product derivation for software product lines. It is based on a set of deltas specifying modifications that are incrementally applied to a core product. The applicability of deltas depends on application conditions over features. This paper presents abstract delta modeling, which explores delta modeling from an abstract, algebraic perspective. Compared to previous work, we take a more flexible approach to conflicts between modifications by introducing the notion of conflict-resolving deltas. Furthermore, we extend our approach to allow the nesting of delta models for increased modularity. We also present conditions on the structure of deltas to ensure unambiguous product generation.