My triggers are often so 'of the moment' that it takes me days/weeks/months after an "indicent" (full-body hallucination, serious flashback, panic attack in public) to work out what triggered me. It can be anything from a single word or seeing something I regard as phallic (a banana, an icy-pole, etc) to something more obscure that relates to a specific memory (a certain species of tree, the feeling of cotton against my bare skin, a favourite song of my abuser, etc...).
As people before me have already mentioned, triggers are so individualised to a person that it is very hard to pinpoint what a trigger is or means. For me, a trigger is something that links very distinctly to a memory; it can differ depending on my mood or emotional state - for example, if I am feeling pretty stable, one trigger (e.g. a man coming close to me) might trigger just one stand-alone memory. But on a bad day, that same trigger might be part of what I describe as a Trigger Train, where one single trigger might result in a single memory, but then that memory will trigger another memory, which will trigger another memory, etc, and it takes a shitload of effort for me to pull myself back into the present moment rather than living within my past.
I'm finding this really hard to explain, so if you have questions or if something doesn't make sense, please ask me. I'm happy to answer you :)