Dear all, I want to use a 12V DC input voltage for my ArduinoUno project. I have found the following transformer, but I am confused if this is a correct voltage: the sec side is defined as 15,3 (which may damage the ArduinoUno) but on the front of the transformer it states that the output is 12V (just what I want).
Measuring the voltage with a multimeter gives 15+V.
My question is: how must I read the specifications on the transformer? Is it ok to use it for my ArduinoUno?
No, don't connect that device to Arduinos.
Likely the output is not regulated, nor stabilized.
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It is probably used to charge a 12V black and decker battery, that's why it says 12V, but the actuall rating is written in the box, it should be DC, you were smart to use the instrument, it always has the final word... Oscilloscope would provide you with more insight on stability, because turning AC to DC creates some oscilations. Uno might be rated to 12V but actually it has a voltage stabiliser rated at 18V, and some capacitors to flatten the curve, problem is that peak to peak 15.5 can become more, the power network can have some transients, and we don't know what's inside the adapter, probably a transformer, while it is likely to work, I would not recommend
Arduino would pull so little current that the voltage would always be close to peaks... Get an adapter, they are everywhere... I mean you can find 12V adapters in dumpsters, use 5V directly on the pins, power it with usb, batteries, not worth risking it...
By the way read on power supply internal resistance, and how the voltage drops with current flowing, also read on how batteries charge, they don't actually charge to rated voltages... IE, car battery holds above 15V but is rated 12V... you will have a clearer picture why the ratings are as such...
If you still want to risk it, turn your voltmeter to measure max, if it can, it will give you a maximum peak value of the voltage, also these ratings are set for 220-240V grid if the grid is a bit higher, your 15.5 will also be a bit higher 
Correct way is always, look on your arduino board and check the power supply stabiliser, check the datasheet for ratings, measure what you have, then you can make an informed decision for yourself, while being aware that 240V network can have quite a bit more voltage sometimes and mitigating for such events...
Thanks Semtex for your detailed answer. I will not use this trasformer for the Arduino and next time I will use the box information to be the truth 
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Hi Railroader, thanks for your answer, I will follow up your advise 