A more thorough explanation:

Term: CA

Definition:

• ca.abbr.CIRCA: Used to indicate an approximate date or time period.
• CA.abbr. CERTIFICATION AUTHORITY: An organization that issues digital certificates to verify the identity of individuals, websites, or other entities on the internet.

Examples:

• The building was constructed ca. 1920.
• The website's SSL certificate was issued by a trusted CA.

The first example illustrates the use of ca. to indicate an approximate date. The second example illustrates the use of CA as an abbreviation for a certification authority, which is an organization that verifies the identity of websites and other entities on the internet.