Generally, the defined
(a) end of the declared takeoff distance - point at which the calculated aircraft flight path has climbed to a nominated height - 35ft (heavy aircraft), 50ft (light aircraft). Gets a bit complicated for heavy aircraft in the detail but, if you have an ATPL as your profile indicates, you have had the usual theory training in the various takeoff distance considerations ...
(b) commencement of the declared landing distance - point at which the calculated aircraft flight path passes 50ft height