X - as part of a botanical name
When you see an 'x' as part of the botanical name of a plant it means it is a hybrid or cultivar. e.g. Cupressocyparis x leylandii. It could have been hybridised manually or naturally as in the case of the Leylandii Cypress.
Xeric
A plant or landscape that conserves water and protects the environment. Most xeric plants will need minimal supplemental watering after an establishment period unless there is an extreme drought.
Xerophytes
Plants adapted to arid climates are called xerophytes and include succulents such as cacti, aloes, and agaves.
Xylem
Vascular tissue that transports water and nutrients from the roots to other parts of the plant.