What does the D in dYdX stand for?

Could you please clarify for me the meaning behind the letter "D" in the name "dYdX"? I'm quite intrigued by the choice of this particular letter and how it might relate to the overall mission or focus of the platform. I'm hoping to gain a deeper understanding of the thought process behind the naming convention, and how it ties in with the values and services offered by dYdX. Thank you in advance for your insight.