For the function f(n)
f(n)=O(g(n))
iff there exist positive constants c and d such that:
f(n) <=c*g(n)
for all n,n>=d.
This is defined to be the worst-time complexity of the function f(n).
For example:
O(n)=3n+2 because,
3n+2 <=4n for all n>=2.   _{Your Comment}