From Bulbapedia, the community-driven Pokémon encyclopedia.
An example of a Pokémon LV.X
Pokémon LV.X are a special kind of Pokémon found in the Pokémon Trading Card Game. They first appeared in the Diamond & Pearl expansion and made their final in-set appearance in the Arceus expansion. Essentially replacing Pokémon-ex, Pokémon LV.X have similar attributes in appearance to their predecessors, with a similar holographic border and similar levels of strength, as well as the same level of rarity. They also lack the major drawback of Pokémon-ex as a player will not receive two Prize cards for Knocking Out a Pokémon LV.X. They do, however, differ greatly in terms of gameplay.
Firstly, Pokémon LV.X are treated as "Pokémon Level-Up" cards as opposed to Basic Pokémon or Evolution cards. The same rules apply as with evolution, however: a Pokémon cannot be Leveled-Up in the same turn as it is played or evolved and a Pokémon that is Leveled-Up will lose all Special Conditions affecting it (though only the Active Pokémon, not the player's Benched Pokémon, can be Leveled-Up). However, a Pokémon LV.X's key trait is that it will still be able to use all Pokémon Powers and attacks usable by the Basic Pokémon or highest Stage Evolution card under it.
As well as that, cards that require the player to search their deck will only allow them to choose a Pokémon LV.X if the card states "Pokémon," "Basic Pokémon or Evolution card" (this applies only to Trainer cards from the EX Series) or "Pokémon LV.X" - cards stating only "Basic Pokémon" or "Evolution cards" (such as Great Ball and Professor Elm's Training Method respectively) will not allow the player to search for a Pokémon LV.X.
Pokémon LV.X cards also share the same name as the Pokémon they Level-Up from, and as such only four of either card are allowed in a 40- or 60-card deck. For instance, a player would be allowed to have two Honchkrow LV.42 and two Honchkrow LV.X in the same deck, but only four Honchkrow would be allowed in total. As well as that, because of the fact that these two cards share the same name, an effect which would remove the highest Stage Evolution card from a Pokémon would, in this example, force the removal of both Honchkrow cards, leaving only Murkrow.
Pokémon LV.X were retired at the end of the Platinum Series of the Trading Card Game, to be replaced in HeartGold & SoulSilver by both Pokémon Prime and Pokémon LEGEND.
Pokémon that have appeared as Pokémon LV.X