I intend to build an online application for defining the meaning of words, synonyms, etc. in a certain language.
This information will come from a combination of printed dictionaries. I'm almost certain this in not (entirely) legal.
However, I'm wondering how to do this without violating copyright. How dictionary.com, synonyms.com, etc. did it?
This reminds me a bit of the process of making business directories. People in that business do often combine, aggregate and edit the work of others. Look at it this way, if you are going to look at every word in those dictionaries, analyze the definitions, combine, rewrite and "add value" to them, then eventually this will become "your work" - especially as you add more sources. On the other hand, if you just slop them together and immediately publish, clearly that is not your work. There is a line between using source material, and plagiarism which I would avoid if I were you.
They have contrqacts with whoever supplies the data.
In most jurisduictions it is not legal to copy databases. In some it IS legal, if you pay someone for making the copy physically (typing) AND if the original data is AS DATA not copyrighted, just as collection (phone book). Most likely not the case for dictionaries.
Talk to the companies creating the original work. Prepare to pay.