If your students are struggling with the complex and often circular definitions found in regular dictionaries, Simple Wiktionary is the way to go.  The definitions are written on about a fifth grade reading level and avoid using technical jargon as much as possible (if jargon has to be used, it’s linked to its own page where it is explained).  Like all other Wikipedia products, Wiktionary is community edited, but because of the thousands of eyes on each definition, errors are harder to sneak by.  There is also a simple version of Wikipedia that uses the same simplified language.

Website: https://simple.wiktionary.org
Subject: Any
Grade Level: 2-12+
Price: Free