BoringSSL is Google's fork of OpenSSL.
BoringSSL is Google's fork of OpenSSL. BoringSSL imports changes from OpenSSL rather than layering Google changes on top of OpenSSL sources. It is a lighter-weight version of OpenSSL that strips out many of the application programing interfaces (APIs) and application binary interfaces (ABIs) found in the latter (reference).
Adam Langley discusses some of Google's design and engineering decisions for BoringSSL on his blog at Imperial Violet.