Saag

The answer to this question is not as simple as it might appear - if we are to believe Wikipedia (and on this website, we generally do).

The word used for spinach in Indian cooking is palak.

Saag, it seems, is a dish, not an ingredient; and as with any dish, different chefs use different recipes. This is reflected in the way Wikipedia's definition keeps changing – Wikipedia being, of course, "the online encyclopedia that anyone can edit". The one I originally picked up (in 2016) was "a leaf-based dish" that includes spinach, but may also include any or all of mustard leaves, fenugreek, and finely-chopped broccoli. Later in 2016 it said "Saag/saj ... can be a catch–all term for various green–leaved dishes", and lists the ingredients as "spinach, mustard leaf, collard greens, basella, etc.".

"Collard greens" are "various loose–leafed cultivars of Brassica oleracea, the species which also contains vegetables including cabbage and broccoli"; basella is a bit more exotic, but includes something called Malabar spinach.

In January 2017, Wikipedia offered two alternative versions: "Saag ... is a leaf-based (spinach, mustard leaf, collard greens, basella, etc.) dish ... made from spinach, mustard leaves, finely chopped broccoli, or other greens ..."

By way of comparison: allrecipes.co.uk says "Saag is a curry of cooked mustard or similar 'bitter' greens (kale, turnip greens), and spinach or similar mild greens (chard, beet greens). Any combination of greens works!"

It has to be said that not many people would answer anything other than "spinach" to this question (assuming they knew what they were talking about). But to be strictly correct, it would appear that something like "leafy green vegetables" should also be allowed as an alternative answer.