The rate of flow of electric charge (negatively-charged electrons.)
When circuit is formed, the current flows from the positive to the negative terminal of a cell.
Current can be measured using an ammeter. — Ammeter has to be connected in series with the component being measured.
Wires in electrical circuits are made of metal because it is a good conductor of electric current.
In the wires, the current is a flow of negatively-charged electrons.
Current is measured in amperes, or, amps (A).
$$ I=\frac{Q}{t} $$
Current is the flow of positive charge. From positive terminal to the negative terminal of a cell.
And this, is known as conventional current.
The opposite direction to electron flow, electrons are negatively charged, so they flow from the negative terminal to the positive terminal of a cell.
TL;DR: Conventional Current — Positive to Negative
Electron Flow — Negative to Positive