Close Combat and Melee are closely related terms, but the Chronicle X rulebook tends to use the term Close Combat to refer to the situation of opposing Models being Adjacent to each other, while Melee refers to the attacks enabled by being in Close Combat.
Melee
Melee Attack refers to the attacks Models and Alien Units can make when they are Adjacent to each other, and have Line of Sight.
Melee attacks use the Melee Attack Combat Value, a value usually accompanied by a fist symbol on the stat cards. Then any effects and modifiers that apply may raise or lower the Combat Value.
Once the combat type and relevant Combat Values have been determined, all Attacks are handled in the same basic way.