ST 6" x 6" x 3/8" steel member???

Is it possible it means square tube?

I agree with jayrod12, I take that to be square tube - so in the US it should be called out as HSS6x6x3/8

"I am looking at a structural detail showing a T-pipe support using a column composed of a ST 6" x 6" x 3/8" member." What does the detail show? A T or a square tube?

Maybe it is a T-Section? But the flange width is only 5.0 inches, not 6.0 inches.

Oengineer:
This seems to be the old common/std. acronym or terminology bugaboo. There is a sq. structural tube which is 6" x 6" x 3/8" which makes a very good canti. column, which seems to be what you’re talking about. And, the details should show the tube. As mentioned above, by Dauwerda, in the U.S. we would call that a HSS 6x6x3/8. The drafter seems to have called it Structural Tube (a ST). Tee sections cut from rolled beams are usually called WT’s or ST’s when cut from WF beams or cut from Std. or S section beams. They are usually called out as WT 6x25 or ST 6x25, or some such, with the first number being the Tee depth and the second number being the weight per foot, and nobody in their right mind would use one of these for a column section.

Terminology...in Australia they are called SHS, square hollow section. Then there are RHS and CHS.

HSS used to be called "TS" back in the day.
Is it even HSS anymore? Isn't there some new lingo around this?

It's also possible they just meant to call out an angle, which would use the dimensional nomenclature, and, depending on the view shown, might look identical on the drawing.
My older AISC books show a "tee" section distinct from the WT and ST sections, and they were referred to by dimensions but 6x6x3/8 was not one of those dimensions, either.

Still HSS in Canada