Certain terms used in describing the burr puzzles have specific meanings:
Rods (also piece, bar or stick) - Rods are the basic parts of which are built burr puzzles. Its length is always greater than the cross-sectional dimension. The most common dimension is the 6x2x2 unit.
Notched Rod - A rod with notches cut out.
Square Rod - A rod whose cross-section is a square
Solid Rod - A rod without notches
Simple notches (Cuts) are notches which can be done using a saw or a dado blade.
Complicated notches (cuts) are notches for which preparation a chisel must be used.
A notchable Rod is a rod which has simple notches.
A non-notchable Rod is a rod that has complicated notches:
Regular Rod - A regular rod is a notched, square rod with notches in multiples of 1x1x1 cubes.
A Burr puzzle - An interlocking construction puzzle that is made of notched rods.
A regular Burr puzzle - A Burr puzzle made of three sets of regular rods that intersect each other at right angles. This puzzle can have internal voids.
Three-piece Burr puzzle - A three piece burr puzzle is made of three rods which has a square cross-section that is intersecting at right angles.
Six-piece Burr puzzle - A six piece burr puzzle is made of three sets having two rods that intersect each other at right angles.
Related terms and definitions
Core - The core of a regular burr is the volume taken up by the intersection of the rods.
Weight - The weight of a regular burr is the number of 1x1 cubes of the core which are occupied by rods.
Solid burr puzzle - For a regular burr puzzle we can say that it is solid if it has a maximum weight, meaning there is no internal voids. Each solid burr puzzle must have a lock rod whose purpose is to secure the interlocking rods of the 3D cross. A lock rod is actually a solid rod because it has no notches. To disassemble this kind of puzzle just draw out the lock rod.