Notion of direct gap and indirect gap
The term "gap" is used in semiconductor physics to describe the valence band and the conduction band of semiconductors.
Between the two bands there is a gap of energy in which a charge carrier cannot be found - this is called a forbidden band. The energy gap between the two bands is known as the gap.
The family of semiconductor materials, insulators with a band gap of around 1eV, can be divided into two groups: materials with a direct gap, such as most of the compounds in columns III and V of the periodic table of chemical elements, and materials with an indirect gap, such as silicium (column IV).
Complément :
The concept of direct and indirect gaps is linked to the representation of the energy dispersion of a semiconductor: the E (energy) - k (wave number) diagram. This diagram is used to spatially define the extrema of the conduction and valence bands. In a semiconductor at equilibrium, these extrema represent energy domains where the density of p-type carriers in the valence band and n-type carriers in the conduction band are high.
A semiconductor is said to have a direct gap when the maximum of the valence band and the minimum of the conduction band are close to the k wave number on the E(k) diagram. Conversely, a semiconductor is said to have an indirect gap when the conduction band minimum and the valence band maximum are at different values from the k wave number on the E(k) diagram.

The conduction band minimum is located on the Δ axis, at k ≠ 0, making it an indirect gap semiconductor.