The first meal of the day, usually eaten in the morning.
You should put more protein in her breakfast so she will grow.
(by extension) A meal consisting of food normally eaten in the morning, which may typically include eggs, sausages, toast, bacon, etc.
We serve breakfast all day.
The celebratory meal served after a wedding (and occasionally after other solemnities e.g. a funeral).
(largely obsolete outside religion) A meal eaten after a period of (now often religious) fasting.