As far as mountain concerned, the current convention is that “highest” means measured from the sea level to summit: “tallest” means measured from the bottom of the mountain to the top.
Mount Everest is the highest mountain in the world but it’s not the tallest.
Mauna Kea is the highest point on the island of Hawaii. The inactive volcano is a modest 4206 m above the sea level, but when measured from the seabed to its summit. It is 10, 200 m about three quarters of a mile taller than Mount Everest.
Mount Everest is Not The Tallest Mountain - KnowledgeNuts. (n.d.). Retrieved May 6, 2015, from http://knowledgenuts.com/2013/07/21/mount-everest-is-not-the-tallest-mountain/