Learn in the textbookPKI and the Core of Public Key Infrastructure
DefinitionA root certificate (ルート証明書) sits at the top of the chain of trust between certificates; it is a (self-signed) certificate that the top-level certificate authority signs for itself.
ExplanationEach certificate is vouched for by a higher certificate authority's signature, and that one by a CA higher still, forming a chain of trust (certificate chain). Certificates of the CAs in between are called intermediate certificates; at the top of the chain no one is left above to vouch for it, so the root certificate is self-signed. Browsers and operating systems come with trusted root certificates already installed.
AnalogyIf a company appointment takes effect only after approval by the section manager, then the department head, then the CEO, the root certificate is the CEO's seal. No one stands above to approve it, yet every employee accepts that seal as genuine from the start.
Memory hook"Root": in a certificate tree drawn upside down, trust flows from the root at the top down to the branches.