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I am a beginner on Machine Learning.
In SVM, the separating hyperplane is defined as $y = w^T x + b$.
Why we say vector $w$ orthogonal to the separating hyperplane?

Geometrically, the vector w is directed orthogonal to the line defined by $w^T x = b$. This can be understood as follows:

First take $b = 0$. Now it is clear that all vectors, $x$, with vanishing inner product with $w$ satisfy this equation, i.e. all vectors orthogonal to w satisfy this equation.

Now translate the hyperplane away from the origin over a vector a. The equation for the plane now becomes: $(x − a)^T w = 0$, i.e. we find that for the offset $b = a^T w$, which is the projection of the vector $a$ onto the vector $w$.

Without loss of generality we may thus choose a perpendicular to the plane, in which case the length $vertvert a vertvert = vert b vert /vertvert wvertvert$ which represents the shortest, orthogonal distance between the origin and the hyperplane.

Hence the vector $w$ is said to be orthogonal to the separating hyperplane.

The reason why $w$ is normal to the hyper-plane is because we define it to be that way:

Suppose that we have a (hyper)plane in 3d space. Let $P_0$ be a point on this plane i.e. $P_0 = x_0, y_0, z_0$. Therefore the vector from the origin $(0,0,0)$ to this point is just $<x_0,y_0,z_0>$. Suppose that we have an arbitrary point $P (x,y,z)$ on the plane. The vector joining $P$ and $P_0$ is then given by:
$$ vecP - vecP_0 = <x-x_0, y-y_0, z-z_0>$$
Note that this vector lies in the plane.

Now let $hatn$ be the normal (orthogonal) vector to the plane. Therefore:
$$ hatn bullet (vecP-vecP_0) = 0$$
Therefore:
$$hatn bullet vecP- hatn bullet vecP_0 = 0$$
Note that $-hatn bullet vecP_0$ is just a number and is equal to $b$ in our case, whereas $hatn$ is just $w$ and $vecP$ is $x$. So by definition, $w$ is orthogonal to the hyperplane.

Using the algebraic definition of a vector being orthogonal to a hyperplane:

$forall x_1, x_2$ on the separating hyperplane,

$$ w^T(x_1-x_2)=(w^Tx_1 + b)-(w^Tx_2 + b)=0-0=0 smallBox.$$

Let the decision boundary be defined as $w^Tx + b = 0$. Consider the points $x_a$ and $x_b$, which lie on the decision boundary. This gives us two equations:

beginequation
w^Tx_a + b = 0 \
w^Tx_b + b = 0
endequation

Subtracting these two equations gives us $w^T.(x_a - x_b) = 0$. Note that the vector $x_a - x_b$ lies on the decision boundary, and it is directed from $x_b$ to $x_a$. Since the dot product $w^T.(x_a - x_b)$ is zero, $w^T$ must be orthogonal to $x_a - x_b$, and in turn, to the decision boundary.