According to Wikipedia : A monogram is a motif made by overlapping or combining two or more letters or other graphemes to form one symbol. Monograms are often made by combining the initials of an individual or a company, used as recognizable symbols or logos. A series of uncombined initials is properly referred to as a cypher (e.g. a royal cypher) and is not a monogram.
Here are 95 Excellent Monogram Logo Designs for inspiration, focusing on monograms in particular.