Co-occurrence in SEO refers to the frequency and proximity with which specific keywords or phrases appear together within a webpage or across a website. It helps search engines understand the contextual relationship between terms and the relevancy of content to specific search queries. In natural language processing (NLP), co-occurrence is used to analyze text data and establish semantic connections between words, which can improve the accuracy of search results and content categorization. This concept is fundamental in semantic search strategies, enhancing content visibility and relevance in search engine rankings.