(physics) A rare stable isotope of neon, ²¹₁₀Ne, having ten protons and eleven neutrons; it amounts to less than 0.3% of the element in nature

The presence of three copies of the 21st chromosome in the karyotype.

(physics) A radioactive isotope of polonium, ²¹⁰₈₄Po, having a half-life of 138 days; it as used as a reference alpha emitter in the calibration of radiation counters

Abbreviation of twenty-first.

plural of 21700

plural of 2170

21700 (electrochemical battery of a secondary storage cell)

2170 (electrochemical battery of a secondary storage cell)

(often with "the") Los Angeles, after the 213 area code.

(often with "the") Manhattan, after the 212 area code.