I came across the term Symbol Clock Frequency in this Intel programmer reference manual (P131). I could not find what the symbol clock frequency actually is in the manual or online and thought it might be worth asking here.
It's the clock at which bytes encoded to 8b10b TMDS symbols are sent over HDMI. For 24-bit resolutions, pixel clock rate matches the symbol clock rate.
Edit: Quote from the linked programmer reference manual:
The PLL output frequencies are 5x the symbol/TMDS rate, which is 1/2 the bit rate.The PLL output is divided by 5 to become the symbol/TMDS clock frequency used in the display engine.
That makes it apparent that symbol rate means the TMDS character rate, and that symbol/TMDS rate is 10x the bit rate.