161

161 is the natural number that follows 160 and precedes 162. In arithmetic, it is a semiprime, being the product of the primes 7 and 23.

In various numeral systems, 161 is written as 10100001 in binary, 241 in octal, A1 in hexadecimal, and CLXI in Roman numerals. In Unicode and many extended character encodings, the decimal value 161 corresponds to the code point U+00A1, the inverted exclamation mark used in Spanish and other languages.

The 161st day of the year falls on June 10 in common (non-leap) years and on June 9 in leap years. This places it in early June in the Northern Hemisphere.

Astronomy and space objects include asteroid 161 Athene, a main-belt asteroid discovered in 1876 by Johann Palisa and named after the Greek goddess Athena.

Historically, the year 161 AD saw the death of Roman Emperor Antoninus Pius and the accession of Marcus Aurelius and his co-emperor Lucius Verus. This year marked the formal transition of power and occurred during ongoing military campaigns, including the Parthian War, which continued into the following years. The period would later be associated with the Antonine Plague, which emerged in subsequent years.

Overall, 161 is characterized by its composite structure, its multiple base representations, its role as a calendar marker, its place in asteroid nomenclature, and its connections to historical events in the Roman Empire.