Self Destruct Command
The Self Destruct command causes the unit to self destruct after a delay. The self destruct timer is 5 seconds, with the exception of bomb units, which self destruct instantly. This command is only accessible via the Ctrl+D hotkey.
Self destruction can be cancelled by issuing the Self Destruct command again during the delay period.
All of the instant-detonating bomb units have a version of Self Destruct called Detonate, accessible via menu button or by pressing D, which detonates them immediately but does not directly affect non-suicide units in the current selection. You can use this to prevent accidental self-destruction of important units.
The Cancel Terraform command on individual terraform blueprint objects has a similar effect to Detonate, with the difference that terraform units don't have any explosions and will disappear silently.