The purpose of a digital signature in online communications is to ensure the authenticity and integrity of digital messages or documents. A digital signature employs cryptographic techniques to create a unique identifier linked to the signer's identity, similar to a handwritten signature. When a recipient receives a signed document, they can verify that it comes from the claimed sender and has not been altered during transmission. This security feature is crucial for e-commerce, legal agreements, and sensitive communications where trust and verification are paramount.