A crawler is a program that visits Web sites and reads their pages and other information in order to create entries for a search engine index.

crawler is google bot made to check all website content.

An internet searcher crawler is a program or mechanized content that peruses the World Wide Web in a deliberate way with a specific end goal to give progressive information to the specific web index.

When you hear people talk about Crawlers in the context of SEO, they are referring to the programs that search engines use to scan and analyze websites in order to determine their importance, and thus their ranking in the results of internet searches for certain keywords.