TFT(cs, dc, rst); // What is dc?

On the page for the TFT constructor the info is missing.

https://www.arduino.cc/en/Reference/TFTConstructor

Perhaps someone could fix it. Thanks

There are several alternative names for the same signal: DC Data / Command RS Register Select A0 Address Select

The TFT library class that comes with Arduino IDE is rubbish (tm) It is better to use the Adafruit_ST7735 library. Install via the Library Manager.

Post a link to the actual screen that you have bought. Then we can suggest the most suitable library.

David.

Already answered well by david_prentice while I was writing this, but since it has some additional information I'll post it anyway:

I submitted a bug report about this last year: https://github.com/arduino-libraries/TFT/issues/8 but unfortunately Arduino has taken no action to fix it. I had originally only reported the problem but this has motivated me to research what "dc" stands for. It turns out it's "data/command select". The Arduino TFT library is really just a wrapper around an outdated version of the Adafruit ST7735 library. I have no clue why Arduino did that. Maybe they thought "ST7735" would be too scary for beginners? The modern version of the Adafruit library is better commented than the outdated one used by Arduino. I had a look at the source code and found this: https://github.com/adafruit/Adafruit-ST7735-Library/blob/master/Adafruit_ST7735.cpp#L21

   @param  dc   Data/Command pin #

The comment in the example sketches gives a little more information: https://github.com/adafruit/Adafruit-ST7735-Library/blob/master/examples/graphicstest/graphicstest.ino#L33

#define TFT_DC        38 // Display data/command select