Thero is an honorific

Honorific

An honorific is a word or expression with connotations conveying esteem or respect when used in addressing or referring to a person. Sometimes, the term is used not quite correctly to refer to an honorary title...

 term for fully ordained Buddhist monks

Bhikkhu

A Bhikkhu or Bhikṣu is an ordained male Buddhist monastic. A female monastic is called a Bhikkhuni Nepali: ). The life of Bhikkhus and Bhikkhunis is governed by a set of rules called the patimokkha within the vinaya's framework of monastic discipline...

 in the Buddhist monastic order. The term Theri is used fully ordained Buddhist nuns

Bhikkhuni

A bhikkhuni or bhikṣuṇī is a fully ordained female Buddhist monastic. Male monastics are called bhikkhus. Both bhikkhunis and bhikkhus live by the vinaya...

. The terms are derived from Pāli language

Pali language

Pāli is a Middle Indo-Aryan language of the Indian subcontinent. It is best known as the language of many of the earliest extant Buddhist scriptures, as collected in the Pāi Canon or Tipitaka, and as the liturgical language of Theravada Buddhism.-Etymology of the name:The word Pali itself...

 and translate as 'elder and venerable'. These terms are used at the end of a monk or nun's other names. They are also useful to distinguish Buddhist monastics from lay people (upāsaka and upāsikā) who keep precepts. Usage of these terms varies in each Buddhist tradition

Schools of Buddhism

Buddhism is an ancient, polyvalent ideological system that originated in the Iron Age Indian subcontinent, referred to variously throughout history by one or more of a myriad of concepts – including, but not limited to any of the following: a Dharmic religion, a philosophy or quasi-philosophical...

. In Sri Lanka

Sri Lanka

Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka is a country off the southern coast of the Indian subcontinent. Known until 1972 as Ceylon , Sri Lanka is an island surrounded by the Indian Ocean, the Gulf of Mannar and the Palk Strait, and lies in the vicinity of India and the...

n Theravada

Theravada

Theravada ; literally, "the Teaching of the Elders" or "the Ancient Teaching", is the oldest surviving Buddhist school. It was founded in India...

 Buddhist tradition, these terms are widely used.

Some prominent theros / theris :

• Arahant Ananda Thera
  Ananda
  Ānanda was one of the principal disciples and a devout attendant of the Buddha. Amongst the Buddha's many disciples, Ānanda had the most retentive memory and most of the suttas in the Sutta Pitaka are attributed to his recollection of the Buddha's teachings during the First Buddhist Council...
  
  
• Arahant Mahapajapati Gotami Theri
  Mahapajapati Gotami
  Mahāpajābatī Gotamī was the first woman to request ordination from the Buddha and to join the Saṅgha...
  
  
• Balangoda Ananda Maitreya Thero
• Kiribathgoda Gnanananda Thero
• Ayaa Tathaaloka Theri
  Ayya Tathaaloka
  [זהו זה] Ayya Tathaaloka Bhikkhuni is an American-born Theravadan nun, scholar and Buddhist teacher. She is also the founder of Dhammadharini Vihara, co-founder of Aranya Bodhi Hermitage, recipient of the 2006 Outstanding Women in Buddhism Award, a presenting scholar at the 2007 International...
  
  
• Gangodawila Soma Thero
  Gangodawila Soma Thero
  Gangodawila Soma Thero was a pioneer Buddhist monk from Sri Lanka. Following tradition, he used the name of his birthplace, Gangodawila, in front of his name...
  
  
• Nyanaponika Thera
  Nyanaponika Thera
  Nyanaponika Thera or Nyaniponika Mahathera was a German-born Sri-Lanka-ordained Theravada monk, co-founder of the Buddhist Publication Society, contemporary author of numerous seminal Theravada books, and teacher of contemporary Western Buddhist leaders such as Bhikkhu Bodhi.-Chronology:*1901: born...
  
  
• Nanavira Thero
  Nanavira Thera
  Ñāṇavīra Thera born Harold Edward Musson was an English Theravāda Buddhist monk, ordained in 1950 in Sri Lanka...