In morphology

Morphology (linguistics)

In linguistics, morphology is the identification, analysis and description, in a language, of the structure of morphemes and other linguistic units, such as words, affixes, parts of speech, intonation/stress, or implied context...

 and syntax

Syntax

In linguistics, syntax is the study of the principles and rules for constructing phrases and sentences in natural languages....

, a clitic is a morpheme

Morpheme

In linguistics, a morpheme is the smallest semantically meaningful unit in a language. The field of study dedicated to morphemes is called morphology. A morpheme is not identical to a word, and the principal difference between the two is that a morpheme may or may not stand alone, whereas a word,...

 that is grammatically independent, but phonologically

Phonology

Phonology is, broadly speaking, the subdiscipline of linguistics concerned with the sounds of language. That is, it is the systematic use of sound to encode meaning in any spoken human language, or the field of linguistics studying this use...

 dependent on another word or phrase. It is pronounced like an affix

Affix

An affix is a morpheme that is attached to a word stem to form a new word. Affixes may be derivational, like English -ness and pre-, or inflectional, like English plural -s and past tense -ed. They are bound morphemes by definition; prefixes and suffixes may be separable affixes...

, but works at the phrase

Phrase

In everyday speech, a phrase may refer to any group of words. In linguistics, a phrase is a group of words which form a constituent and so function as a single unit in the syntax of a sentence. A phrase is lower on the grammatical hierarchy than a clause....

 level. For example, the word an in the phrase:

an apple

Clitics may belong to any grammatical category, though they are commonly pronoun

Pronoun

In linguistics and grammar, a pronoun is a pro-form that substitutes for a noun , such as, in English, the words it and he...

s, determiners, or adposition

Adposition

Prepositions are a grammatically distinct class of words whose most central members characteristically express spatial relations or serve to mark various syntactic functions and semantic roles...

s.
Note that orthography

Orthography

The orthography of a language specifies a standardized way of using a specific writing system to write the language. Where more than one writing system is used for a language, for example Kurdish, Uyghur, Serbian or Inuktitut, there can be more than one orthography...

 is not a good guide for identifying clitics: clitics may be written as independent words, bound affixes, or separated by special characters (e.g., apostrophe).
There are a number of different clitics depending on their position in relation to the word that they are connected to.
A proclitic appears before its host.

• English
  English language
  English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...
  
  : an apple
• French
  French language
  French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...
  
  : Je t'aime.

"I you-love." = "I love you."

• Ancient Greek
  Ancient Greek
  Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
  
  : ὁ ἄνθρωπος

"the person"

An enclitic appears after its host.

• Latin
  Latin
  Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
  
  : Senatus Populusque Romanus
  SPQR
  SPQR is an initialism from a Latin phrase, Senatus Populusque Romanus , referring to the government of the ancient Roman Republic, and used as an official emblem of the modern day comune of Rome...
  
  

"Senate people-and Roman" = "The Senate and Roman people"

• Ancient Greek
  Ancient Greek
  Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
  
  : ánthrōpoí (te) theoí te

"people (and) gods and" = "(both) men and gods"

A mesoclitic appears between the stem

Word stem

In linguistics, a stem is a part of a word. The term is used with slightly different meanings.In one usage, a stem is a form to which affixes can be attached. Thus, in this usage, the English word friendships contains the stem friend, to which the derivational suffix -ship is attached to form a new...

 of the host and other affixes.

• Portuguese
  Portuguese language
  Portuguese is a Romance language that arose in the medieval Kingdom of Galicia, nowadays Galicia and Northern Portugal. The southern part of the Kingdom of Galicia became independent as the County of Portugal in 1095...
  
  : Ela levá-lo-ia.

"She take-it-COND

Conditional mood

In linguistics, the conditional mood is the inflectional form of the verb used in the independent clause of a conditional sentence to refer to a hypothetical state of affairs, or an uncertain event, that is contingent on another set of circumstances...

." = "She would take it."

The endoclitic splits apart the root and is inserted between the two pieces.