The symbol that the author actually intended to appear there is:
It means that what is of one side of it is defined as what is on the other side. Its ‘home’ is in formal logic, where the expressions on both sides of it would be formulated by using logical symbols, as in, for example:
Once such a definition is introduced, it can be invoked in any proofs that follow, as the ground for treating the expressions on the two sides of the symbol as interchangeable.
Professional philosophers, when writing for philosophically trained audiences, which can be presumed to be familiar with the symbol, sometimes use it informally to connect ordinary English expressions, and that was done in the example that the OP encountered.
The symbol looks like an equals sign with def or df. above the top line. It, however, occasionally happened in the past that the typesetters were not equipped to render this symbol properly; they then resorted to placing df. next to the equals sign, which is what the OP has seen. While that may have been the best available substitute under the circumstances, it is confusing, because it may leave an impression that df. is a part of the expression on the right side of the symbol, rather than a part of the symbol. The problem should not appear in anything that is published today, as the present-day software makes it easy to render the symbol correctly.