I am getting difficulty in understanding the definition of the statistic.
From wikipedia, I come to understand that statistic is any 'information' (for example, range, mean, variance) of any sample of any given population.
Whereas in my college the definition of statistic is given as:
Suppose $(X_1, X_2,... ,X_i,...,X_n)$ is a random sample of size $n$ from any given PDF or PMF. A function $T=t(X_1, X_2,... ,X_i,...,X_n)$ free from unknown parameter is called a statistic.
I cannot understand the definition given in my college. Are the two definitions related to each other? What is the need of finding the statistic?