Definition of Swing high & Swing Low:

swing high: recent highest or equal high bar with either side bars as lower highs. 
(whenever equal high appear we chose recent equal high as swing high bar)

swing low: recent lowest or equal low bar with either side bars as higher lows. 
(whenever equal low appear we chose recent equal low as swing low bar)

NOTE: actually it is simple 3bar swing i.e. one left (i+1) and one right side (i-1) to 
swing high bar(i) which is called as middle bar. (so total 3bars to qualify as swing)