The 2017–18 South-West Indian Ocean cyclone season was below-average. The South-West Indian Ocean basin is defined as the portion of the Indian Ocean that is west of 90°E and south of the Equator. For the majority of the basin, the season officially began on November 15, 2017, and officially ended on April 30, 2018; for Mauritius and the Seychelles, however, the season ended on May 15, 2018. The first storm of the season, Tropical Cyclone Ava, formed on December 27, 2017. The final storm of the season, Severe Tropical Storm Flamboyan, dissipated on May 1, 2018.
Throughout the season, nine tropical depressions formed. Eight of these depressions were eventually classified as moderate tropical storms. All of these moderate tropical storms were eventually classified as severe tropical storms. Five of these severe tropical storms, Ava, Irving, Berguitta, Cebile, and Dumazile, were eventually classified as tropical cyclones. Two of these tropical cyclones, Berguitta and Cebile, were eventually classified as intense tropical cyclones.
The South-West Indian Ocean classifies tropical disturbances and tropical cyclones using seven categories: tropical disturbance, tropical depression, moderate tropical storm, severe tropical storm, tropical cyclone, intense tropical cyclone, and very intense tropical cyclone. Tropical disturbances have 10-minute sustained winds of <28 knots (<50 km/h). Tropical depressions have 10-minute sustained winds of 28-33 knots (51-62 km/h). Moderate tropical storms have 10-minute sustained winds of 34-47 knots (63-88 km/h). Severe tropical storms have 10-minute sustained winds of 48-63 knots (89-117 km/h). Tropical cyclones have 10-minute sustained winds of 64-89 knots (118-165 km/h). Intense tropical cyclones have 10-minute sustained winds of 90-115 knots (166-212 km/h). Very intense tropical cyclones have 10-minute sustained winds of >115 knots (>212 km/h).