Transition Metal
Definition
The block of metals in the middle of the periodic table: iron, copper, gold, and friends. They are known for colorful compounds and multiple possible charges.
Technical definition
Transition metals are the d-block elements in groups 3-12 with partially filled d subshells. They typically form colored compounds, show multiple oxidation states, act as catalysts, and are generally hard, dense metals with high melting points.
Example
Iron can be Fe^2+ (pale green) or Fe^3+ (yellow-brown): that variable charge is classic transition-metal behavior, and copper sulfate's deep blue is another signature.
Zinc, cadmium, and mercury sit in the d-block but are often excluded from transition metal in the strict sense because their d subshells are full, though many textbooks include them loosely.