Constitution (previously known as Hitpoints) is the skill that controls how healthy and physically enduring your character is. You can use your Runescape Gold to higher your Constitution level, the more damage your character can take either from combat, traps, or any other dangerous or harmful thing before succumbing to death.
Less of a skill on its own, Constitution could better be defined as an attribute, due to the inability to train it solely, instead relying on gaining experience at the same time as training various other, combat related, skills.
You can travel to the Hospital area in the Duel Arena and purchase the Constitution Skillcape from Surgeon General Tafani for 99,000 rs gold if you have finally reached level 99 in the skill, . This will enable you to use a boost that raises your Constitution level to 100 and also allows you to perform the Constitution Skillcape Emote. The cape itself is members only. However, free players can still perform the emote granted they are wearing the cape.
If Constitution is your first skill to reach level 99, you will be rewarded the Constitution cape. However, if you have other skills at level 99, you will be awarded the Constitution cape (t). If you have the untrimmed version of the cape and achieve level 99 in another skill, the cape will become trimmed automatically.