Baby Spinach
Baby spinach is young spinach with tender leaves and a mild green flavor. It wilts quickly in hot dishes and can also be used raw in salads and bowls.
Identity
Tender greens
Wilts fast and adds mild green flavor.
Add baby spinach near the end of cooking when you want it just wilted. Because the leaves shrink a lot, a large raw handful becomes a much smaller cooked amount.
Wikimedia Commons
Reference image
Baby Spinach reference image.

Source: Wikimedia Commons
Used In Recipes
Substitutions
Acceptable
Baby Spinach -> Baby Kale
1 pound (16 ounces) fresh baby spinach or baby kale, or frozen spinach or kale, or 1 ½ pounds Tuscan kale or spinach bunches, washed and stems removed
The source lists this as an alternative in the same ingredient row.
Source: Best Stuffed Shells