The thick unrefined sugar syrup from the processing of cane or beet into sugar. The word was formery applied to all types of sugar syrup, including Golden Syrup, but In most of the English-speaking world it is used now for the almost-black syrup also called 'molasses'.
In its earliest usages the word was used to mean plant extracts in general, appearing in English as early as the 14th Century 'Ayenbite of Inwyt'. In this sense it occurs in some early translations of the bible where more modern versions use 'balm', as in Chapter 8, v.22 of the Book of Jeremiah in Coverdale's 1535 Bible; "I am heuy and abashed, for there is no more Triacle at Galaad".
The modern usage is known at least since a receipt in 'The Queene-Like Closet' (1672) by Hannah Woolley (Wooley 1672) for 'Treacle Wine'.
Treacle has been used, mixed with ammonium nitrate, as a commercial high explosive.
