for a large sample size, the sampling distribution of x mean is approximately normal, irrespective of the shape of the population distribution. The sample size is usually considered to be large if n>=30 .
CENTRAL LIMIT THEOREM
Meaning of CENTRAL LIMIT THEOREM in English
Sean Woo's Finance, GIS and real estate English glossary. Английский глоссарий по ГИС(Географическая информационная система) и недвижимости Sean Woo's Finance. 2012