A basha is an army term to make a shelter, like most army phrases, slang we took from service overseas for example dobi is a Indian word for laundry, chit is I.d card or pass from the French chit an lieu, laarger we coined from the Boer war and means to form your wagons in a circle for defence and we use this phase when mechanised or on foot. Basha comes from the Malay campaign back in the 50s and means shelter.
We used to be issued a olive green poncho which had eyelets on the edge to attach bungees or paracord, they were just big enough to cover two people in a shell scrape. Guys used to buy oz ponchos (they were olive green in the day and couldn't be worn) because they were larger had better eyelets. We also bought civvy made camouflage ones. Then the army got wise and started to issue decent ponchos (cant be worn) which was camouflaged, bigger and had loops along the sides as well as eyelets, so it doubled up as a quick stretcher for casevac. A tarp is the same thing as an army basha, just different companies gear towards civvies making large or small and usually not as strong as one used by a daft soldier