It could be a black widow spider if it is small and thin-legged with a shiny round black abdomen and red markings under it. If it's fuzzy with a red spot on it's back, and the front part of the body is more-or-less the same size as back part, then it's a black jumping spider.
If the stripe is rather large and the abdomen is longer rather than spherical, it's probably an Ant Mimic Spider (in the family Clubionidae, genus Castianeira). And it's harmless. If the spider is super shiny and has a very round abdomen, then it's probably some sort of widow and you should stay away.
The peacock jumping spider (Maratus volans) is known for its colorful markings on its abdomen that resemble the pattern of a peacock feather. It has a small body size that can resemble a walking pea due to its round shape and bright colors. This species is native to Australia.
A black spotted yellow spider that just ate a gumball.
The spider you are describing is likely a common orb-weaver spider of the genus Araneus. These spiders are generally harmless to humans and are known for their distinctive round abdomen and black dot on their underside. They are commonly found in gardens and wooded areas.
The top of a black widow spider is typically shiny black and features a distinctive red hourglass shape on its abdomen, which is one of its most recognizable characteristics. The legs are long and slender, and the body is round and shiny.
A black widow. This spider is only a black widow if the entire spider is black, with a round-ball looking back-body, female variety if there is a red hourglass shape marking on the underside of the spider. From the top black widows are completely black.
A black widow spider is black with a red hourglass shape on its abdomen, although the coloration can vary. Females are larger than males, with a body length of about 1.5 inches. They are known for their venomous bite.
Wolf Spider
A male black widow has orange and black stripes. Heres my description of black widows. This is about both male and female black widow spiders. It is shiny and coal black in colour. The female averages 8-10 mm's in length and has long slender legs and a round abdomen. Usually on the underside of the female's abdomen is a red hourglass mark and one or two red spots over the spinnerets and along the middle of her back. The male is 3-4 mm's long with an elongated abdomen. The male's legs are larger then the females and each joint is orange brown and black on the ends. On the sides of the male's abdomen there are 4 pairs of red and white stripes. Young black widows are orange, brown and white. They acquire their black clothing with age.
A green garden spider, also known as the orb-weaver spider, has a bright green body with yellow spots or markings. It also has a large, round abdomen and long, skinny legs. This spider is known for its intricate orb-shaped webs.
The kind of spider with a large round body is typically a tarantula.