The term Mleccha is used in Sanskrit often.

There are over fifteen definitions,right from Vedas,Vyakaranas to Amarakosha.

The descriptions vary.

Mlecchas are people who do not follow Vedic life; are uncouth;lack in culture;Speak unintelligibly: Barbarians.

Considering the fact that Bharatavarsha covered more or less the whole world in ancient times,there were only a few countries which were out side Bharata Varsha.

They were called as Mlecchas being outside Vedic Culture.

Some of them were so uncouth,they were called Barbarians.They were not entitled to perform Parvana Sraddha.

Their lands were called Bhoga Bhoomi,land of enjoyment as against Bharatavarsha, which is Karma Bhoomi,where performances rites earn them Punya and Paapa,Good results and Sin.

Some of these lands,Kambhoja embraced Hinduism.

The definition of Milechchcha.

‘Mleccha (from Vedic Sanskrit mleccha, meaning “non-Vedic”, “barbarian”), also spelled Mlechchha or Maleccha, is a name, which referred to people of foreign extraction in ancient India. Mleccha was used by the ancient Indians originally to indicate the uncouth and incomprehensible speech of foreigners and then extended to their unfamiliar behaviour, and also used as a derogatory term in the sense of “impure and/or “inferior” people.

In ancient India, this term was also applied by the ancient Indian kingdoms to foreigners. The word Mleccha was commonly used for ‘outer barbarians of whatever race or colour’.

The Indians referred to all alien cultures that were less civilized in ancient times as ‘Mlechcha’ or barbarians. Among the tribes termed Mlechcha were Sakas, Hunas, Yavanas, Kambojas, Pahlavas, Bahlikas and Rishikas. The Amarakosha described the Kiratas, Khasas and Pulindas as the Mleccha-jatis. Indo-Greeks, Scythians,and Kushanas were also mlecchas

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tatra mlecChaan pulindaan ca shuurasenaan tathaiva ca |
prasthaalaan bharataan caiva kuruum ca saha madrakaiH || || 4-43-11
kaa.mboja yavanaan caiva shakaan pattanaani ca |
anviikshya daradaan caiva himavantam vicinvatha || 4-43-12

11, 12. tatra= there – in north; mlecChaan pulindaan ca= Mleccha-s, Pulinda-s – provinces of; tathaiva= like that; shuurasenaan ca= Shurashena, also; prasthaalaan bharataan caiva= Prasthala, Bharata, also, thus; madrakaiH saha = Madraka, along with; kuruum ca= Kuru, also; kaamboja yavanaan caiva= Kaambhoja, Yavana [countries,] also, thus; shakaan pattanaani ca= of Shaka, cities, also; daradaan caiva= Darada, also, thus; anviikshya= on scrutinizing; himavantam vicinvatha= at Himavanta [Himalayas,] search out.

“There in the north, the provinces of Mleccha-s, Pulinda-s, that way Shurashena – Prasthala – Bharata – Kuru – Madraka – Kaambhoja – Yavana shall be scrutinized along with the cities of Shaka and Darada, and then search in Himalayas. [4-43-11,12]

‘The Mleccha is the province of the then India to the North-West and it is defined as: prati anto mleccha sy˜t – amara koþa – go m˜Õsa bhakÿako yastu viruddham bahu bh˜ÿate sarva ˜c˜ra vihŸna× ca mleccha itiu abhidŸyate – bodh˜yana ‘at the end of the country there is Mleccha province…amarakosha, ‘eaters of beef, talkers of odd languages, devoid of all ethics [with reference to Indian scriptural ethics, especially marriage as an institution, immoralities etc.,] and they are called Mleccha-s…’ Bodhaayana aphorisms….,’

Mlecchas.

Definition from other sources.

Purāṇa

1a) Mleccha (म्लेच्छ).—Born out of the left side of Vena’s body when the latter’s body was churned;¹ begin with Anu, son of Yayāti; ruled by Dakṣa; ruled by Turvasu; ruled by the hundred sons of Pracetas of Druhyu family in the east; the rule of the kings;not to be seen in Kuśadvīpa; Śakas (Parādas, Viṣṇu-purāṇa), Pallavas (Paplava, Viṣṇu-purāṇa) and Kāmbojas, all Mlecchas; (Ābhiras, Viṣṇu-purāṇa) Guruṇdas and Vṛṣalas, also belonged to this group; kingdoms of, found in the lotus coming out of the naval of the Lord;their kingdoms side by side with those of the Āryas;people who live in their countries are ineligible for Pārvana śrāddha;attain salvation at Benares;do not molest the women of their enemies;to be conquered by Kalki; put down by Pramati god.

1b) These were the elder Madhucchandasas, who were cursed by Viśvāmitra to become Mlecchas;¹ defeated by Bharata; these were the Mlecchas of the north;worship progenitors; eleven kings ruled for 300 years. Then Kolikilas followed; their dharma described.

(Source): Cologne Digital Sanskrit Dictionaries: The Purana Index

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Dharmaśāstra (religious law)

Mleccha (म्लेच्छ) refers to “persons whose language is not intelligible”. The word is used throughout Dharmaśāstra literature such as the Manusmṛti. (also see the Manubhāṣyaverse 7.149)

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Vyākaraṇa (Sanskrit grammar)

Mleccha (म्लेच्छ).—(I) a word although correct, yet looked upon as incorrect owing to its faulty utterance; (2) a person like the uncultured people, who is not able to pronounce words correctly म्लेच्छा मा भूमेत्यध्येयं व्याकरणम् (mlecchā mā bhūmetyadhyeyaṃ vyākaraṇam) I. 1, Ahnika 1.

Mleccha referred to people of foreign extraction in ancient India. The Sanskrit term Mleccha, referring to the indistinct speech of some non-Aryans. Mleccha is used for one who is impure, dirty or uncultured. It is derived from the root mlich~mlech, meaning to speak indistinctly (like a foreigner or barbarian who does not speak Sanskrit). We find the use of root also in Mahābhāṣya.

Some explanations of the name “mleccha” suggest that the word was derived from the Indo-Aryan perception of the speech of the indigenous peoples. Namely, “mlech” was a word that meant “to speak indistinctly.” As such, some suggest that the Indo-Aryans used an onomatopoeic sound to imitate the harshness of alien tongue and to indicate incomprehension, thus coming up with “mleccha”.

Mleccha (from Vedic Sanskrit म्लेच्छ mleccha, meaning “non-Vedic” or “non-Aryan”, “barbarian”, “foreigners”), also spelt Mlechchha or Mlechha.

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( Source. https://www.wisdomlib.org/definition/mleccha