• One of the ordinal numbers: First, Second, Third ...
• angle measuring 1/3600th of a degree, then usually referred to as an "arcsecond"
• The base unit of time in the International System (S.I.) of Units that is equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.