Vanilla (バニラ Banira) is a term used in various trading card games that refer to cards without any abilities.
In Z/X -Zillions of enemy X- TCG, vanilla are Z/X that has no ability in its card text, this includes any kind of startup, automatic, and continuous ability. These kind of Z/X are listed as Z/X with "no ability" (能力なし Nōryoku Nashi) in the official card database.
While these Z/X does not seems to provide any direct advantage compared to Z/X with ability, they possess high Power which is enough to outclass most Z/X of the same Cost in battle. With few exceptions, Z/X with no ability also sets the highest possible printed Power for Z/X of their Cost without any kind of limitations or drawback, which is [ Cost x 1500 + 500 ].
As of the latest set, there are no Cost 1 or Cost 8 and higher vanilla.
Other than the aforementioned definition, Z/X with only a single keyword ability (Start Card, Life Recovery or Void Bringer) and no other abilities are also referred as vanilla. Free Cards with time-limited ability (e.g. "This ability is available until...") often also treated as such. However, as they still has some kind of ability in its card text, they are not "truly" vanilla.