In circuit analysis, the dot convention is a convention

Convention (norm)

A convention is a set of agreed, stipulated or generally accepted standards, norms, social norms or criteria, often taking the form of a custom....

 used to denote the voltage polarity of two mutually inductive components, such as winding on a transformer

Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

.

The polarity

Electrical polarity

Electrical polarity is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only...

 of all dotted terminals will be the same at any particular time, assuming an ideal transformer with no leakage inductance

Leakage inductance

Leakage inductance is the property of an electrical transformer that causes a winding to appear to have some inductance in series with the mutually-coupled transformer windings...

.

When the current in a loop enters the dot, then positive voltage is induced at the dot of the other loop.
Alternately, when current in a loop leaves the dot, negative voltage is induced at the dot of the other loop.
Similarly if two inductors are in series the dot convention can be used in the same manner as in case of transformer.
For sinusoidal AC such as the mains, the convention can be thought of in either of the following ways:

1. The current
   Electric current
   Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
   
    going into one dot (either dot) "tries" to come out the other dot.