Sadaqah is Arabic for charity. In the modern context, this means voluntary charity, given in addition to obligatory alms-giving, zakat.
Sadaqah is very wide term and used in the Qur’an to cover all kinds of charity and alms-giving:
“Those who believe, and do deeds of righteousness, and establish regular prayers and regular charity, will have their reward with their Lord: on them shall be no fear, nor shall they grieve” (Qur’an: 2 : 277).
Even a kind word and smile can be considered sadaqah.