Shiva or Siva? Which is the original name?

Can we say Siva instead of Shiva ? Many say Om nama chivaya, or Siva Siva. Why? Is it correct? 

Shiva is a God worshipped by all Hindus. Both North and South Indians lay claim to this deity. How old is His worship? Where did He belong? I have tried to answer this question by delving into the meaning of the word and the antiquity.

0:00 Intro

1:55 Method of Determination

2:44 Shivah meaning

4:59 Shivah Definition

5:45 Shivah etymology

7:54 Reference shooks

8:34 Shivah in Vedas

9:44 Summary Shivah

10:33 Siva in Ancient Tamizh

12:12 Names of ancient Siva

13:32 Siva Etymology

14:49 Shiva Rudra Siva