Thai Holy basil (Ocimum sanctum) is also know as Tulsi in India and is a herb that is very distinctive in the aroma it imparts to dishes. It’s sharp and peppery, with perhaps a touch of lemon and these flavours are enhanced by quick cooking. Many people confuse it with Thai Sweet Basil (horapa in Thai), which is an altogether different herb.
How to distinguish Thai Holy Basil from impersonators? Look out for the reddish, purple stems.
Why is it called Holy Basil? Because it is an important herb in Hindu ceremonial practices, “Tulsi” means The Incomparable One in Sanskrit.
How is Holy Basil used? Besides for religious purposes as mentioned above, it is a very popular herb in some South East Asian cuisines, for eg, it gives the defining flavour in Thai Basil Chicken Recipe (Pad Kra Pao Gai | ผัดกระเพราไก่).
Substitutes: the bottom line – NONE! If you can’t find it, you can use alternative herbs like Thai Sweet Basil or Mint but the resulting dish will be a different one.