SEN - Special Educational Needs (SEN) is a legal term. It describes the needs of a child who has a difficulty or disability which makes learning harder for them than for other children their age.
Around one in five children has SEN at some point during their school years. Some children have SEN right through their time in school.
SEN covers a broad spectrum of difficulty or disability. Children may have wide-ranging or specific problems. Eg, a child might have difficulty with one area of learning, such as letters or numbers. Or they might have problems relating to other children, or to adults.