Zakat is the obligatory share; sadaqa is voluntary giving beyond it.
What it means
Zakat is a fixed duty on wealth. Sadaqa is any charity, in money or in kindness, and the Prophet said that even a smile to your brother is sadaqa (al-Tirmidhi). The Quran praises those who give “secretly and openly” (2:274), and says that charity given to please God is for the giver’s own benefit (2:272).
Term and Arabic as listed in Ismail al Faruqi, Toward Islamic English (International Institute of Islamic Thought, 1995), p. 35, listed as ṢADAQA. The definition is restated here.