Are Stop orders triggered by the bid or ask price?

The trigger price for stop orders depends on the product.

For FX spot: Stop to buy is triggered by the Bid price. Stop to sell is triggered by the Ask price. 

For index CFDs, futures CFDs: Stop to buy is triggered by the Ask price. Stop to sell is triggered by the Bid price. 

For stock CFDs, stocks, ETFs, futures: Stops are triggered by the Last Traded price.

For listed options: navigate to the following article to learn more: Stop orders on exchange traded options