Well, "load" can be anything from a power to a mass if we're talking about what you can load in the back of your truck, so it's a fuzzy term. Its meaning is context-dependent.
I'm interested in the wiring inside a home, and voltage inside homes here is fixed (in my case 220-240 V AC)
OK. The maximum amount of current that can flow through a wire of a certain cross section (like 2.5 mm2) is determined by temperature rise.
If your 2.5 mm2 wire was insulated with enamel (withstands heat well) and in the airflow of a fan, it could carry 50 amps without trouble, probably a lot more.
The wires you'll put in your walls are insulated with PVC, which has a rather low melting point, and they are inside conduits with no airflow. Also the conduits are often inside in-wall insulation like fiberglass. Under such circumstances, they must be very conservatively rated for current, hence the limit for 2.5mm2 is 16A when using fuses, or 20A with circuit breakers, which are more accurate.
So, I was just saying that you define "overload" by selecting the circuit breaker current rating. It's up to you. It should at least protect the wires, and sometimes also the load. For example, if the load is an air extraction fan which draws 20W, it is wise to put in a 2A circuit breaker, because it will trip if the fan is clogged by dust and stalls, whereas a 16A breaker will let the fan catch fire.
current equals load
Not really, consider a motor for example, it isn't a resistor so it will draw reactive current. If current and voltage are not in phase, you'll have more current that what you get by calculating I=P/V. There is also the power factor. In this case the circuit breaker and wires must be sized according to the current, thus the apparent power in VA, not in Watts.
Also, motors tend to have huge starting currents (which are not overload), huge stall currents (which will result in a fire if the breaker doesn't trip), and mechanical load-dependent current draw (the power factor also changes according to load).
So to define "overload" for a motor is a bit complex. It isn't just "overcurrent" because that happens every time the motor starts. So you'd pick a circuit breaker with an accurate current limit (to trip on stall) but a slow reaction time (to allow the motor to start) like this one:
Notice it has an adjustment knob, so you can set the trip current according to the motor's documentation.