Apex of heart - Apex cordis
The Apex (apex cordis) is directed downward, forward, and to the left, and is overlapped by the left lung and pleura: it lies behind the fifth left intercostal space, 8 to 9 cm. from the mid-sternal line, or about 4 cm. below and 2 mm. to the medial side of the left mammary papilla.
This definition incorporates text from a public domain edition of Gray's Anatomy (20th U.S. edition of Gray's Anatomy of the Human Body, published in 1918 – from http://www.bartleby.com/107/).