a season is a consecutive runs... nichijou was 1 season, existing out of 2 cours...
The Japanese have sometimes subdivided television series and dramas into kūru (クール?), from the French term "cours" for "course", which is a 3-month period usually of 13 episodes. Each kūru generally has its own opening and ending image sequence and song, recordings of which are often sold. A six-month period of 26 episodes is also used for subdivision in some television series.
Anime is shown of Japanese TV, if you look at the TV schedules you will notice that the majority of new anime series start at the beginning of January, April, July or October. Due to this, the common denominator tends to be 13 (52 weeks in the year, divided by the four seasons), which means the most series are 13, 26 or 52 weeks long.
Due to national holidays (such as Golden Week, New Year, etc.), it is not uncommon for a fair number of series to be only 12, 24, 25, 50, 51 episodes long because they skip a week or two during the run.
some anime show said 12~ ep's was a half season while double that 24~ was a full season
might have been oreimo, when kirino received word her adaption went from full to half season I'd say a sweet spot would be ~15 eps, so it could give a 12 ep season room to breath a bit
however i hope online services gain more popularity so it could be a prime place to broadcast, and more dosh for the industry.
All hail that one background character that has no dialogue or value to the plot and still goes super saiyan for half of a split second for literally no reason whatsoever.