2,4,5 Trichlorophenol (thing)

Short for 2,4,5,trichlorophenol, the active ingredient in many defoliant chemicals, notably Agent Orange:

        Cl
          \____
          //  \\ 
      OH_//    \\_Cl
         \      /
          \====/
               \
                Cl

2,4,5-T's greatest danger comes not from the chemical itself (bad enough as it is), but because its manufacturing process leaves the chemical contaminated by dioxin.