The letter M in a circle, commonly used in to identify a Metro (subway, underground train) station in a city area.
Circled M was approved as part of Unicode 1.1 in 1993 under the name “Circled Latin Capital Letter M” and added to Emoji 1.0 in 2015.
U+24C2
U+FE0F
Ⓜ️ Circled latin capital letter m
Ⓜ️ Variation selector-16