“Semi” means half. We are using a logarithmic scale in half the possible places, that is, on one out of two axes. That was true with our natural base logarithmic plotting as well, but standard usage assigns the name “semilog” only to base `10` logarithmic plotting. For purposes of finding model functions, it doesn't matter much which base you use. The primary virtue of base `10` is enabling direct visual estimates of function and data values.