Messier 91 (also known as NGC 4548 or M91) is a barred spiral galaxy located in the Coma Berenices constellation and is part of the Virgo Cluster of galaxies. M91 is about 63 million light-years away from the earth. It was the last of a group of eight nebulae discovered by Charles Messierin 1781.

Originally M91 was a missing Messier object in the catalogue as the result of a bookkeeping mistake by Messier. It was not until 1969 that amateur astronomer William C. Williams realized that M91 was NGC 4548, which was documented by William Herschel in 1784 (according to other sources, however, the nearby spiral galaxy NGC 4571 was also considered as a candidate for Messier 91 by him.)