Trishul or Trident.

AUM.

Trishul (Trident) is the main weapon of Lord Shiva and that's why his follower Siddha's are seen with this weapon. 

Tree + Shul = Tree means 3 and shul means fork. 

It has great symbolic meaning too. 

Trishul represents SAT, TAT and AUM. 

SAT is Truth or Absolute Bliss, TAT Universal intelligence or Consciousness. 

AUM symbol of GOD, Cosmic Intelligent Creative Vibration. 

In another word it represents the three aspects of consciousness – waking, dreaming and sleeping. 

Trishul (Trident) also represents the three gunas – satva , rajas and tamas.  

The three attributes of Nature: Tamas, Rajas and Sattav are Obstruction, Activity and Expansion or Mass, Energy and Intelligence.

In man the three gunas express themselves as Ignorance or inertia, Activity or Struggle and Wisdom.

This has always been and continue to be present in all things and beings in the world. 

Holding a trishul (Trident) signifies that Shiva (the divinity) is above all the three states – waking, dreaming and sleeping. 

Yet Shiva  is the upholder of these three states. 
It also helps its followers from Dahik या दैहिक, Physical, Davik या दैविक any curse and Bhautik भौतिक - Physical Tapa - Energy.