OneLook lets you perform reverse lookups. This means that you can type words that you expect to be part of the definition of a term, and terms whose definition contains those words will be listed in order of match quality.
This can be used in a way that's pretty close in practice to just typing a description of the word you're seeking, and the site actually encourages using it that way ("this tool lets you describe a concept and get back a list of words and phrases related to that concept").
However, you must not expect any actual understanding of "concept": no natural language processing is involved, which could narrow down the results considerably but also fail to yield anything meaningful in a majority of cases, and would also be much more difficult to implement. This is ultimately just an unusual but simple type of dictionary search.