filters.website_ids. Website lookups are free.
Search by name or domain
websites.search matches a retailer name or domain and returns up to limit websites (default 5, max 20), best match first.
List websites
websites.list pages through supported websites, up to the top 5,000. Pass category_ids to list the websites that sell those categories, best first. Iterating the result fetches later pages for you.
Get a website by ID or domain
GET /v1/websites/{website_id} accepts a website ID or a domain. It is REST-only for now; the SDKs add it in the next major version.
websites.retrieve (GET /v0/websites) still resolves a URL to a website, but is deprecated in favor of the endpoints above.
Website filters restrict the retailers whose offers can match. They are different from brands; see Website.