SHA-1 (Secure Hash Algorithm 1) is a cryptographic hash function that can convert an arbitrarily long string of data into a digest with a fixed size of 160 bits. This digest is commonly displayed as a 40 character hexadecimal number.
The SHA-1 algorithm is now considered insecure. SHA-1 certificates are no longer in compliance with the CA/B Forum Baseline Requirements, or supported by the current versions of major web browsers.
The Secure Hash Algorithm (SHA) series of hash functions is comprised of different sets (SHA-0, SHA-1, SHA-2, SHA-3).