And can one species with no subspecies (like the very lowest branch of the phylogenetic tree with no subbranches) be a clade or a taxon?
In: 3
A clade is a group of organisms that share a common ancestor and all its descendants. A taxon is a group of organisms that are classified together based on some criteria. Not all taxa are clades, but all clades are taxa.
*Taxon* is basically just scientific Greek for *category* or *group.* Any time you sort things into types, the types are taxa.
A clade is the kind of taxon that modern biology feels is most appropriate for categorizing organisms: a group consisting of all and only the descendants of some prior organism or species.
A taxon is much more general, and refers to any group that taxonomists place organisms into. A clade is that, plus the organisms share a common ancestor. A clade is a type of taxon, specifically a monophyletic taxon.