Diagram of simple filtration: oversize particles in the feed cannot pass through the lattice structure of the filter, while fluid and small particles pass through, becoming filtrate.
An odd function can be identified by replacing both the x and y values with -x and -y values. If the values in your equation are the opposite (positives are negatives and negatives are positives), then the function is odd. Take, for example, this equation y = x^2.
Historically the concept was defined only for natural numbers. These a days the set of all integers multiplied by 2 is considered the set of even numbers, i.e. $\dots,-4,-2,0,2,4,\dots$ n and all the integers not in that set are defined to be odd.