Transmission gate vs triac
A triac cannot be used as an analogue switch for signals if you wish to maintain the quality of the signals because a triac will naturally block the signal below around 1 or 2 volts peak-to-peak even when the triac is supposedly being enabled via its gate. In other words a triac is highly non-linearly to small signals.
A triac finds its home in the control of power wave-forms i.e. switching on or off loads that care more about grunt-power and dozens to many thousands of amps than with pure sine wave quality.
A transmission gate (and there are many to choose from) is suitable for most analogue situations where small to medium signals (micro volts to tens of volts) can be switched on or off with very little distortion being introduced.
A transmission gate I would definitely call an analogue switch but I wouldn't use this term for a triac.