These words refer to downward movement. Descend suggests slow or gradual movement, often down an incline: mountain climbers descending from their overnight camp near the peak’s summit; an aeroplane descending for a landing. Drop and fall , by contrast, suggest a sudden and sharp downward movement. Fall may refer to any such movement regardless of how it is caused; drop most strictly suggests something released or set free at a height: a misstep that sent him falling down the stairs; dropping food by parachute to the stranded survivors. This distinction is not always adhered to, however, in the case of drop : Prices dropped drastically during the last hour of heavy trading.

Sink refers to direct downward movement through water or air rather than along an incline. The word often suggests a relatively slow or gradual fall , although this is not always the case: watching the boat fill with water and finally sink ; the sun sinks in the West; cold air sinking into the low-lying valley; iron bars that sank quickly without a bubble. Sink may also suggest the fading away of energy or consciousness or a gradual decrease in volume: She sank into a coma; the tide sinking back along the beach. Decline is exclusively concerned with this last sense of sink, specifically pointing to a dying down or running out of something: wondering how long it would be before the fever began to decline ; a recession in which prices slowly declined . Decline would most appropriately suggest a gradual lessening, failing or waning. If a sharp or sudden downward source were indicated, drop or fall would render this meaning less ambiguously.