Soberness is defined as having a blood alcohol concentration (BAC) of 0.00, not being under the influence of any illicit drugs, and being in a proper state to enforce your duty of care to alcohol recipients. According to the Merriam-Webster dictionary, the definition of sober is not intoxication or abstaining from intoxicating drugs or drinking alcohol. Being sober can also mean refraining from using any addictive substance. This definition was developed by Black's Law Dictionary, 2nd ed.
Some common synonyms for sober are serious, sedate, solemn, and grave. While all of these words don't necessarily mean light or frivolous, sober emphasizes the seriousness of purpose and the absence of frivolity or levity.