Metallic Character

The metallic character of an element is how reactive an element is in regards to giving up an electron in a chemical reaction.  Metals tend to lose electrons because of their low ionization energies and tend not to attract electrons due to their low electronegativities.  Thus, the most metallic elements will be those that have large atomic radii and low ionization energies.  Hence, metallic character is a periodic trend that increases as one goes down and to the left of the periodic table.  Conversely, non-metals in the upper-right of the periodic table have high electronegativities and high ionization energies and have a non-metallic character.  

The metallicity of an element is not that discrete of a property, but is actually more continuous, as there are elements in-between that are metalloids/poor metals such as silicon that exhibit properties of metals and non-metals.  Hydrogen is a unique exception that exhibits metallic properties at extremely high pressures and low temperatures.  Other properties of metals include shininess, hardness, durability, etc.