A subdomain is a prefix added before a site's main domain that creates what functions as a distinct address: given zeo.org as the main domain, blog.zeo.org or shop.zeo.org are subdomains, each capable of running entirely separate content, even a different platform or technology stack, from the main site.
Search engines generally treat a subdomain as its own distinct entity for ranking and authority purposes, related to the main domain, but not automatically inheriting its full authority the way a subfolder (zeo.org/blog) would. That's the core tradeoff against subfolders: a subdomain offers more technical independence, useful when a section genuinely needs different infrastructure, a help center on different software, an app on a different stack, but it generally builds its own separate authority more slowly than the same content would under a subfolder of the established main domain. Most current SEO guidance leans toward subfolders by default for content meant to reinforce the main site's authority, reserving subdomains for cases where the technical separation is genuinely necessary rather than just organizational preference.
