Dictionary.com just got a lot worse. For example, looking up helium gives you entries from four dictionaries, labeled "American", "British", "Scientific", and "Cultural", but nowhere on the page can one find the names of the dictionaries the entries are from.
Some, but not all, pages are suffixed with:
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
But the content in the "British" tab clearly matches that of the Collins English Dictionary, yet the word "Collins" is nowhere to be found. I'd be surprised if this was consistent with the licensing agreement they had with HarperCollins (or whoever) that allowed the re-publication.
Internet Archive (you might have to turn JavaScript off) reveals the content in the "Scientific" and "Cultural" tabs is from The American Heritage Science Dictionary and The New Dictionary of Cultural Literacy, respectively, but it seems preposterous to have to do this every time just to learn the source of a definition (and it could become increasingly difficult as they might add, remove, or swap dictionaries).
So you might want to refrain from citing Dictionary.com, especially the content under tabs other than "American" (which is presumably always Dictionary.com Unabridged, which is based on the Random House Webster's Unabridged Dictionary), while this problem persists. I'd be surprised if not revealing the source was consistent with the agreement they had with the licensers, so hopefully it won't be long.