What is SPLADE and how does it differ from BM25?

SPLADE, or Sparse Lexical and Expansion model, is a learned sparse retrieval model that produces sparse vectors where each dimension corresponds to a vocabulary term, like BM25, but with learned weights and query expansion via masked language modeling. SPLADE can add terms not present in the original text, such as synonyms or related concepts, closing the vocabulary gap that hurts BM25. It combines the interpretability and exact-match strengths of sparse retrieval with some of the semantic understanding that dense embeddings provide.