I've seen the word "spike" used in relation to source-control.
For example, a project I'm working with has three top-level folders: trunk, branches and spikes.
What's the meaning of this term?
"Spike" is a synonym for "Tag". It is a "spike" in the ground meaning it doesn't move versus a branch, which grows.
For further clarification, a 'tag' or 'spike' represents a release version of your project. Tags are snapshots of that release and as such are not actively developed on though branches can be created from a tag and then further developed.