Bird's eye chili or "Thai chili" is a chili pepper

Chili pepper

Chili pepper is the fruit of plants from the genus Capsicum, members of the nightshade family, Solanaceae. The term in British English and in Australia, New Zealand, India, Malaysia and other Asian countries is just chilli without pepper.Chili peppers originated in the Americas...

 of the species Capsicum frutescens

Capsicum frutescens

Capsicum frutescens is a species of chili pepper that includes the following cultivar and varieties:*Piri piri, also called African Bird's Eye or African devil* Kambuzi pepper, Malawian pepper*Malagueta pepper*Tabasco pepper, used to make Tabasco sauce...

L. in the family Solanaceae

Solanaceae

Solanaceae are a family of flowering plants that include a number of important agricultural crops as well as many toxic plants. The name of the family comes from the Latin Solanum "the nightshade plant", but the further etymology of that word is unclear...

, commonly found in South-east Asia. It can also be found in India

India

India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

, mainly Kerala

Kerala

or Keralam is an Indian state located on the Malabar coast of south-west India. It was created on 1 November 1956 by the States Reorganisation Act by combining various Malayalam speaking regions....

, where it is used in traditional dishes of the Kerala cuisine . This species (known as කොච්චි (kochchi) in sinhalese) is also found in rural areas of Sri Lanka, where it is used as a substitute for green chillies. It is also a main ingredient in kochchi sambal, a salad made using freshly scraped coconut ground with thai chillies and seasoned with salt and lime juice.

The term Bird's eye chili is also used for the North American Chiltepin pepper

Chiltepin pepper

Capsicum annuum var. glabriusculum is a variety of Capsicum annuum that is native to southern North America and northern South America. Common names include chiltepin, chiltepe, and chile tepin. This variety is the most likely progenitor of the domesticated C. annuum var...

, both due to their small round shape and because they're widely spread by birds.
The bird's eye chili plant is a perennial with small, tapering fruits, often 2-3, at a node.