noun

• (typography) A unit of measurement equal to the height of the type in use.
  "That the price of work, paid for by letters be advanced from fourpence to fourpence halfpenny per thousand, including English and brevier; and, in leaded matter, the ems and ens at the beginnings and ends of the lines not to be reckoned in the width."

pron

• (rare, nonstandard) A gender-neutral third-person singular object pronoun, the objective case of ey or e, equivalent to the singular them and coordinate with him and her.
  "If the author uses such notation, it should be up to Em to indicate Eir intentions clearly, but there’s no harm checking first."

intj

• (Scotland, Ireland) a form of hesitant speech, or an expression of uncertainty
  "She was abused by, em... David, I think. That was his name, he's a real em... what's the word, narcissist. You should really stay away from him."

noun

• (medicine) Initialism of emergency medicine.
• (sciences, countable) Abbreviation of electron microscope.
• (sciences, uncountable) Abbreviation of electron microscopy.
• (uncountable) Initialism of electromagnetism.