(algebraic topology) A homomorphism that operates on the kth boundary element.

An internal interface that separates neighbouring misoriented single crystals in a polycrystalline solid.

(Australian rules football) A match official who decides when the ball has over the boundary line (out of bounds), and whether it was on the full from a kick. (Under the rules as of 2007, a free kick results from on the full, or a throw-in otherwise.)

(Australia) A person employed to travel along the boundary of a station, to check and maintain the fencing.

plural of boundary-rider

A storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air, with a similar effect to a cold front.

plural of boundary rider

plural of surface boundary layer

(meteorology) The portion of the atmosphere lying next to the surface of the earth and extending up to between 50 and 100 meters.

The geological signature of the transition between the Cretaceous period and the Tertiary period 65.5 million years ago.