What is laminectomy defect mean?

Old surgery. 'laminectomy defect' just means prior surgery or laminectomy seen on a imaging study (usually an mri).
Prior surgery. The report is describing a focal absence of bone in the back of the spine typically due to prior surgery (laminectomy) where is is removed. Occasionally this finding may due to failure for that portion of bone to develop normally.
Post surgical? https://www.google.com/search?q=lamina+spine&hl=en&rlz=1c1avsu_enus360us360&prmd=imvns&tbm=isch&tbo=u&source=univ&sa=x&ei=u2_nt_iles6j2awaobmwaw&ved=0chyqsaq&biw=1068&bih=599 lamina are part of the "posterior elements" of your spine, which surround the spinal cord proper. Laminectomy defect is likely post surgical, probably an area through which they operated in order to get to the central canal.