Sounds stupid but I have a reason to ask this question. I have a power adapter that generates 5.7 Volts DC 300 mA and I wonder if I can use it on the 5V Pin of an Arduino Nano? All datasheets always say 5 Volts to this pin but there is no range given. Could this adapter work or will it destroy the Nano?
Recommended operating voltage range for the ATmega328p is 1.8~5.5v - 5.7 is a hair too high. The limits for the USB-serial chip are probably similar (depending on whether you have official or clone nano there may be differences in the limit for Vcc of the USB-serial adapter). You are, I think, likely to be within the absolute maximum ratings of these parts, but outside the recommended operating range. And some things you might want to connect could be damaged by 5.7v (5.5v "absolute maximum" is not uncommon for 5v parts).
You can fix this real easy though - just put a diode (something common like 1N4001 (or other 1N400x)) in series (ie, power supply + to diode to 5v pin, with band towards the 5v pin) - they have a ~0.7v drop, so you should get ~5v out.
If you don't have diodes, ebay has your back - "Rectifier Diode Assortment" gets you lots of assortment packs of common diodes, starts at like $3.50-4 for 100pc kits shipped from asia, $5-6 for the same kits shipped from USA. This also works for other cheap, simple components (eg, resistors, leds, capacitors) - at those prices, every arduino experimenter should have an assortment of resistors, capacitors, and diodes on hand - for the price of a frappucino, you can have all the resistor and capacitor values you're likely to need on hand 
@DrAzzy: O man! Sometimes I don't see the trees when staring at a forest! You are a genius. I had not thought of a simple diode! Of course I have a whole sortiment of them (as well as resistors and capacitors) around. And yes, 0.7 Voltage drop equals exactly one diode. Thanx for your quick answer! Have a great day!
(Have you actually measured the output voltage of your supply? It it's an "unregulated" supply rated 5.7V @ 300mA, the actual output voltage at the low currents consumed by an arduino might be significantly higher than 5.7V)