Job security

How will I define job security? Not in ways most people define it. Not by the company and whether they have a reputation of firing people in bad times. Not by a comfortable government job( this is probably how my grandparents would!)
I am secured in my job if my work and what I contribute to the company cannot be replicated by anyone on this planet. Will my job give me the opportunity to provide that uniqueness? I hope so.
To make my point clearer I will give an example of two jobs which I do on campus. One in which I write for in the paper is unique, I am pretty sure no one can replicate my column. I bring my own unique perspective when I do gadget/ website reviews.. something which I believe is difficult to find. I have ideas to make the science page grow and opinions that will make sense.
On the contrary my other job is that of an assistant to a project manager. Most part of my job can be easily done by someone else. Its not rocket science and I haven't brought about something individualistic to it as I have brought to the other job. The only part which makes the job feel like this is mine, is when I am helping undergraduate students solve their problems. Maybe there I bring something of different value to my work.
Job security depends on an individual's contribution to his work, for most part. It is never defined by companies or economies.