A default attack is when you attack with your weapon(s) and no skill related animation is executed. Obviously when a WPS is rolled, then you see its animation instead. 
The skills medea mentioned generally qualify, except for Cadence’s third strike. Primal Strike does not, but while it is on cooldown you use your default attack. You also find suitable attack replacers on items like the Mistborn Talisman.
It does not matter which button or hotkey you assign the attack to.