Keyword difficulty in SEO refers to a metric that indicates how challenging it would be to rank for a specific keyword in search engine results pages (SERPs). This metric helps SEO professionals assess the competitiveness of a keyword, guiding them in choosing target keywords that balance search volume and the effort required to rank.

Several factors contribute to keyword difficulty, including:

1. Domain Authority: The strength and reputation of the websites currently ranking for that keyword.
2. Page Authority: The quality and relevance of the pages competing for the keyword.
3. Content Quality: How well the existing content matches the search intent of users.
4. Backlinks: The number and quality of backlinks to the pages already ranking.
5. SERP Features: The presence of SERP features like snippets, images, or videos, which can affect click-through rates.

Tools like Ahrefs, SEMrush, and Moz provide keyword difficulty scores, typically on a scale from 0 to 100, where higher numbers indicate more competition and lower numbers signify easier opportunities.