"Unripe tomatoes" refers to tomatoes that are not fully grown or mature yet. They are still green and not ready to be eaten or used for cooking until they become ripe and turn red. Full definition
For non-US readers, substitute the tomatillo with unripe tomatoes and perhaps add a squeeze of lime juice to mimic the tomatillos signature tartness. (arthurstreetkitchen.com)
Just remember to avoid letting your cat eat unripe tomatoes, or the leaves and stems of the tomato plant, which can result in tomatine toxicity. (petconsider.com)
If you are using unripe tomatoes in place of tomatillos, remember to add more citrus (lime) to the tomatoes to inject some tartness. (arthurstreetkitchen.com)