A “domain” usually means one of two things, depending on context:

  1. On the internet / websites
    • A domain is the human‑readable address of a site, like example.com or myshop.org.
 * It often has parts:
   * Top‑level domain (TLD): `.com`, `.org`, `.de`.
   * Second‑level: the main name, e.g., `example` in `example.com`.
   * Optional subdomain: `www` or `blog` in `www.example.com` or `blog.example.com`.
 * Behind the scenes, it maps to an IP address via DNS so people can type a name instead of numbers.
  1. In general English
    • A domain can be any area where someone has control, ownership, or special expertise, like “her domain is physics” or “the king’s domain.”

If you tell me the sentence or situation where you saw “what domain,” I can explain which of these meanings applies and phrase it for your post.