This refers to the famous Greek-English Lexicon published by Oxford University Press and originally edited by Henry George Liddell, Robert Scott, Henry Stuart Jones and Roderick McKenzie in the mid-nineteenth century. It is commonly referred to simply as 'Liddell and Scott', following the academic practice of referring to books by their author's or editor's names rather than by their titles.
The Liddell and Scott is still in print today. The ninth edition of the book, which was published in 1940, is freely available online via the Perseus Project. The most recent edition was published in 2007; the Supplement keeps readers abreast of changes and corrections made to the text.