1) Clop 2) Footstep 3) Step 1) Footstep 2) Sound 3) Sound of a step 4) Step 
== Plot == ===Plot setup=== The book depicts the arrival of members of an alien species called the Fithp that have traveled to our solar system from Alpha Centauri in a large spacecraft driven by a Bussard ramjet. The aliens are intent on taking over the Earth. Physically, the Fithp resemble man-sized, quadrupedal elephants with multiple trunks. T...
The number of people visiting a shop or a chain of shops in a period of time....more on Footfall
• (n.) A setting down of the foot; a footstep; the sound of a footstep.
This is a retail term meaning the number of shoppers entering a store or shopping mall.
The extent or measure of numbers of people who visit a business or shop or other retail/leisure/entertainment venue during a given period of time. Footfall is a crucial factor in retailing methods, and also in promotion and advertising which focuses on the physical presence - on foot - of consumers at a particular location
Foot'fall` noun A setting down of the foot; a footstep; the sound of a footstep. Shak. « Seraphim, whose footfalls tinkled on the tufted floor.» Poe. 
[n] - the sound of a step of someone walking
Consumer traffic into a store, or a specific area of a store.
footstep noun the sound of a step of someone walking; `he heard footsteps on the porch`
a footstep. · the sound of footsteps: She heard a footfall on the stairs.