The term bubble in economics means rapid increase of an asset value and followed by sudden contraction or decrease in value, one of the biggest asset bubble is the dutch tulip and the dot-com bubble. There are lots of different opinions about Bitcoin some people compare it to the past bubbles because its value quickly increase and will soon crash but for me Bitcoin is not a bubble, its value is rapidly increasing because more people are adopting it and Bitcoin has a limited supplies. The only way it will collapse is when people lose interest which is very unlikely to happen.