Bionic is an implementation of the C standard library, developed by Google for its Android operating system. It differs from the GNU C Library (glibc) in being designed for devices with less memory and processor power than a typical Linux system. It is a combination of new code and code from FreeBSD, NetBSD, and OpenBSD released under a BSD license, rather than glibc, which uses the GNU Lesser General Public License. This difference was important in the early days of Android, when static linking was common, and since Bionic has its own application binary interface, it can't be replaced by a different libc without breaking all existing apps.
Bionic is a C library for use with the Linux kernel, and provides libc, libdl, and libm (libpthread functionality is part of libc, not a separate library as on some other systems). This differs from the BSD C libraries that Bionic shares code with, because they require a BSD kernel.