Senegal's flag is a green, yellow, and red tricolor with a green star at its center. It has not changed since 1960.
Adopted in 1960
The flag was adopted on August 20, 1960, a few months after independence from France. Decolonization was sweeping across Africa at the time, and each new state needed symbols that owed nothing to the departing colonial power.
The design was settled during the presidency of Léopold Sédar Senghor, one of the leading figures of the independence movement. Other proposals were considered, some drawing on regional groups within Senegal and others on Pan-African ideas, before the current arrangement was chosen.
The three bands and the star
Green stands for Islam, the majority religion in Senegal, and also for progress and hope. Yellow is for wealth and for the natural resources of the country, and by extension for optimism about its economy. Red is for the independence struggle and the people who died in it.
The five-pointed green star in the yellow band stands for unity, equality, and freedom. Sitting at the center, it is the element the rest of the design is built around.
The color scheme is Pan-African, shared with much of the continent and chosen deliberately as a statement of solidarity.
Usage
The flag is raised at government buildings and schools, on national holidays such as Independence Day, and at diplomatic events and sporting competitions where Senegal competes. When flown in a group it takes the position of prominence.
There are no officially recognized variants for particular events or organizations, though the colors turn up unofficially in art and in celebrations, and the flag appears in music and literature as shorthand for the country itself.
Similar flags or influences
Senegal's flag resembles those of Guinea and Mali, which use the same Pan-African green, yellow, and red. The similarity is deliberate and regional. The green star is what separates Senegal's from the others.
References
[1] Government of Senegal. "National Symbols and History." senegal.gov.sn
[2] International Vexillological Congress. "Symbolism and Significance in African Flags." Proceedings 2019.