unleavened bread: flat bread baked without yeast, so it does not rise
Matstsah is bread made and baked without any leaven (no yeast or fermented dough), so it stays flat and dense instead of rising. In the everyday world of the Hebrew Bible it is the bread you make in a hurry, when there is no time to let dough ferment and rise: Lot bakes it for his two guests at evening (Genesis 19:3), and Gideon brings out unleavened cakes for the visitor under the oak (Judges 6:19-21). Because it can be thrown together fast, it is the natural bread of an unexpected meal.
That same speed is why it became the bread of the Passover and the Feast of Unleavened Bread. Israel left Egypt so quickly that the dough had no time to rise (Exodus 12:39), and ever after the people eat flat bread for seven days to remember that night, deliberately purging all leaven from the house (Exodus 12:15, Deuteronomy 16:3). The word names the bread itself; the festival is named separately by adding the word for feast.
Unleavened bread is also the required bread of the altar. Grain offerings and the bread that accompanies certain sacrifices had to be made without leaven (Leviticus 2:4-5, Leviticus 6:16), and the priests ate it in the holy place (Leviticus 10:12). Throughout, it is one and the same food: plain flat bread, set apart by the absence of yeast rather than by any difference in what it is.