Legality means the things that are coming under the control of government, or we can say the thing is legal because the government has control over it. So in the case of Bitcoin, I said it is illegal (in governmental affairs) because Bitcoin is based on a decentralized system which is beyond the power of the government, or the government has zero control over it, and in this case, some governments declare it illegal. Also, governments are afraid of it because it will crash their national currency, and that's why some governments don't allow it in this country. But now things are changing, and governments are understanding its benefits, and they have legalized it. It will take some more time for other states to make it legal, and soon the whole world will use Bitcoin as a global currency.