In literature, a motif is any recurring element that has symbolic significance. For example, it could be a thing, a place, or a statement. The green light in The Great Gatsby is a motif, as is the repeated statement, "My father said that the reason for living is getting ready to stay dead" in William Faulkner's As I Lay Dying.
In biochemistry, a motif is a recurring pattern in the sequence of nucleotides or amino acids — see sequence motif (a.k.a. DNA motif) or structural motif (a.k.a. protein motif).