Trassi (terasi, or blachan) is an extremely strong smelling shrimp paste. The smell disappears during cooking. It is an essential ingredient in most Indonesian dishes. Available in blocks, which need to be sliced thinly and then toasted after which they can be ground up and used as instructed. Also available ready ground.
Trassi is quite pungent so make sure to store it in the refrigerator in a very tightly sealed container.
You can purchase the ground terasi at Indofoodstore.com.
If you don't have terasi then the best substitute is: