According to Wikipedia,
In a plasma, particle velocity can be characterized using a probability distribution. If the plasma is thermalized, the distribution looks like a bell curve, or maxwellian distribution. In this case, it is useful to use the average particle cross section over the velocity distribution. This is entered into the volumetric fusion rate: $$P_{fusion} = n_An_B \langle \sigma v_{A,B}\rangle E_{fusion}$$
Where apparently $\langle \sigma v_{A,B}\rangle$ is
the cross section of that reaction, average over all the velocities of the two species $v$
To my knowledge, anything in between angle brackets denote an inner product. What is $\langle \sigma v_{A,B}\rangle$ expressed explicitly as given its notation?