What is the meaning of USDD?

Could you kindly enlighten me on the significance of USDD? I've heard this term mentioned frequently in the realm of cryptocurrency and finance, but I'm not quite sure what it stands for or what its function is within this domain. Could you please elaborate on its definition, purpose, and any key characteristics that might be pertinent to understanding its role in the cryptocurrency ecosystem? Your explanation would greatly assist me in gaining a clearer comprehension of this term. Thank you for your assistance.