(physics) An artificially produced radioactive isotope of potassium, ⁴³₁₉K, having a half-life of 22 hours; used in medicine as a tracer

(physics) A minor stable isotope of calcium, ⁴³₂₀Ca, having twenty protons and twenty-three neutrons; it amounts to about 0.1% of the element in nature

(biochemistry, uncountable) a protein found in neurons, a component of the exocyst involved in regulating actin polymerization

(UK) Solitary confinement imposed to protect a vulnerable prisoner from other inmates.

(US) George Walker Bush, the 43rd president of the United States, distinguished from his father the 41st president.

Abbreviation of forty-third.