Mods note: I've added "definitions" to the title of this thread.
Never seen it before either. It appears that Google is testing a new vertical channel... by further refining definitions.
These all seem to be dictionary sites, and the listing is clearly a definition, so I tried it for some other words.... first taking a cue from "tenacious" and trying "persistent", which also returns a definition, also in a small font, along with the same four websites, displayed as you say like mini-sitelinks, all on one line.
The dictionary sites Google is currently using appear to be...
Dictionary.com - Answers.com - Merriam-Webster - The Free Dictionary
Google has gotten pretty good at spotting queries for definitions, and these grouped definitions seem to be returned for those queries in particular which don't have many commonly-searched associations.
To test out some words, I googled [free dictionary], went to its Thesaurus section, was presented with a list of 40 random words, and tried some of them out in Google search. In very brief testing, some words I've found that return the definition with grouped suggestions are... "persistent", "vicinity", "concerted", "vicissitudes"
When you try (multisyllabic) words that don't return the grouped definitions, you can generally figure out why. Eg, "metropolitan"
have other associations... Metropolitan Museum of Art, Frequently Asked Questions, etc.
One more ongoing Google serps refinement. Will be interesting to see if this one sticks.
A key question to ask as Google continues to use site content in serps... is this a trade-off for these sites, where featuring them might offer them more traffic, or does presenting the definition on the serps page cost these sites traffic? If the latter, does Google pay a use fee of some sort?