To close a window (typically by clicking a button marked with an X symbol).
To cross out with a series of the letter X, or with scribbles or lines.
To reduce to nothing, to destroy, to eradicate.
To render null and void; to abrogate.
To react with antimatter, producing gamma radiation and (for higher-mass reactants, especially composite particles such as protons) lighter particles (such as pions, muons, and neutrinos).