It's strange that I Google searched and found no answer because even Microsoft has asked their community for opinions on changing this sometimes confusing initialism.
SDL = Secure Development Lifecycle
SDLC = Software Development Lifecycle (supported by Microsoft)
But my question is really about what are the differences between the two lifecycles? That is the question that I was surprised not to easily find in a Google search. It's like I am the first person, ever, to ask this question. LOL
I think I know the difference, but I don't want to influence my thoughts in this question.
Thank you. :)