Celsius is a unit of measurement for temperature, named after the Swedish astronomer Anders Celsius. Although it would be easy enough to say Anders Celsius invented the scale, it’s really named in his honor as it was changed almost immediately upon his death.
Why changed? For reasons understood most clearly by Celsius himself and few (if any) others in the scientific community, he insisted on setting the scale so that 0 °C was the temperature of boiling water and 100 °C was the freezing point of water. If you find the idea of the scale going down to indicate increasing heat and going up to indicate decreasing heat an uncomfortable one to contemplate, you’re certainly not alone.
Not only does it seem strange to us in the present to think of a rising temperature reading as indicative of a falling actual temperature, Celsius’ contemporaries felt the same. Almost immediately after his death in 1744, the Swedish botanist Carolus Linnaeus promptly inverted the scale to its present configuration (wherein water freezes at 0 °C and boils at 100 °C). Other scientists quickly adopted the new and more palatable arrangement, and over two centuries later, we’re still using the scale in such a fashion.