Incorrectly clustering of outlier (outgroup) organisms due to shared dissimilarity to the main group.
Phylogenetic trees are reconstructed based on sequence similarities between organisms. But if a set of similar organisms contains few organisms that are very different from the main group, these outgroup organisms cluster together even though they are very distinct to each other.
Long branch attraction (LBA) means that outgroup organisms are not clustered due to similarity among them, they are clustered due to shared dissimilarity to the main group of organisms. LBA is a systematic error in clustering of sequences and does not show true evolutionary relationships.