Final answer:
A derived character shared among two or more lineages is called a synapomorphy, which is essential in understanding evolutionary relationships as it indicates a trait from a common ancestor.
Step-by-step explanation:
A derived character that is shared among two or more lineages is also referred to as a synapomorphy. This term is used to describe a trait that originated from a common ancestor of the two or more lineages and helps elucidate the evolutionary relationships between them. It's different from a plesiomorphy, which is a trait inherited from distant ancestors, shared by a wider group of organisms, and thus less useful in determining recent evolutionary changes. On the other hand, a clade is a monophyletic group consisting of a single ancestor and all its descendants, while an apomorphy refers to a characteristic that's derived within a single lineage, distinguishing that lineage from its ancestors and other lineages.
In a phylogenetic tree, a clade is represented by a monophyletic group, which includes all and only the descendants from a common ancestor (Answer to the reference question: d. clade).