Extended definition
The amount of resources Googlebot allocates to crawling your site. On small sites (under 10,000 pages) it's not a problem; on large sites, it is.
Context and application
Crawl budget is a function of two variables: “crawl rate limit” (how fast Googlebot can send requests without breaking the server) and “crawl demand” (how much Google wants to crawl your site, based on popularity + freshness). For e-commerce with 100,000+ pages and frequently changing products, optimizing crawl budget becomes critical. Tactics: Disallow on filters with useless parameters, consolidate thin content, eliminate redirect chains, segmented sitemap.
In practice
optimize crawl budget by removing useless pages, redirect chains, 4xx errors.
Related terms
Crawl, SEO Audit.