Initialism of single-lens reflex, or clipping of SLR camera.
"The photographer packed his slr into the bag to take clear shots at the gallery opening."
In plain English: SLR stands for single-lens reflex, which is an older type of camera where you look through the lens to take pictures instead of using a separate viewfinder.
"He reached into his bag to grab an extra slr for the road trip."
Initialism of sorry, late reply.
"He replied 'SLR' in his text message to explain why he wasn't responding sooner."