The word Siva signifies him who controls everything and whom none can control.
The word Siva is given another interpretation. The syllable 'S' means Permanent Bliss. The letter 'i’ means Purusa (the primordial male energy). The syllable 'va' means Sakti (the primordial female energy).A harmonious compound of these syllables is Siva. The devotee shall likewise make his own soul a harmonious whole and worship Siva.
Siva also means pure.
(These above meanings are as explained by Sage Suta).
The name Siva blazing like the forest conflagration reduces mountainous heaps of great sins to ashes without difficulty. True, it is undoubtedly true. (Lord Visnu)
Persons scorched and distressed by the conflagration of sins must drink the nectar of Siva's names. Without that there is no peace and tranquility to those who are scorched and distressed by the sin's wild fire. (Lord Visnu)
The sins which are the root of worldly existence are destroyed certainly by the by the axe of Siva's names (Lord Visnu)
Namah Sivaya means "obeisance to Siva".
Om Namah Sivaya has the same meaning "obeisance to Siva but is prefixed with the mantra OM.