Original: In the late 19th century it came to mean the concept that natural selection was the sole mechanism of evolution, in contrast to Lamarckism, then around 1900 it was eclipsed by Mendelism until the modern evolutionary synthesis unified Darwin's and Gregor Mendel's ideas.
Simplified: In the late 19th century it came to mean the concept that natural selection was the only mechanism of evolution, in contrast to Lamarckism.