Lemma and headword have similar uses in linguistics.  Words are organised in the dictionary according to the lemma or headword.  The lemma or headword is used as the citation form of the word to find its rightful place in the dictionary.  The headword and lemma are sometimes known at the catchword and this is the word that can be used in an encyclopaedia to guide a researcher to the correct entry in the encyclopaedia.  The headword or lemma are arranged in alphabetical order.  The dictionary entry may include alternative meanings of the word, its pronunciation and other  information surrounding the headword.  Lemma and headword are closely related in terms of putting together citations and base words for classification and order of words in the dictionary.  However, the word lemma has other options for its usage in the English language and is found in more subjects and language uses than a headword.

Definition of Headword

A headword is defined as a word at the beginning of an entry made in a dictionary or encyclopaedia or thesaurus.   A headword is also known as a keyword at the beginning of an entry into a glossary or reference of work.  It is a word that may be modified by the addition of an adjunct, or additional words to give meaning to the sentence. The headword, or head, is the word that is necessary to sustain the core meaning of the phrase it is used in.  The headword, a noun, can be modified with determiners adjectives and modifiers adverbs, but essentially the headword must remain as the constant in the phrase and part of the sentence. 

 The headword therefore, is the word at the beginning of an entry in a reference book or dictionary, or it is the word that holds the core meaning of a phrase in a sentence.

Definition of a Lemma

In Lexicography, the art of compiling dictionaries, a lemma is the dictionary form or citation of a set of words that indicate the first word in alphabetical order to be entered into the dictionary.  In this context it is also known as the head word.  For example the word walk, walks, walking, are forms of the same lexeme with walk as the lemma and that is how they are cited in the index.  The lemma is known as the ‘stem word’ because other words branch off from the lemma word.  A lemma is a subsidiary proposition proving another proposition known as a helping theorem.  A lemma is also important for use in lemmatisation, a process used for computational linguistics.  Recognising lemma forms helps with semantic text analysis.  The study of the meaning and use of words.   

Usage of headwords and lemma

Although headwords and lemma do have the same definition in terms of their dictionary or reference to text use, each of these definitions can be taken further, especially in the case of lemma.  Headword appears to be more singular in its definition as a word that heads a reference word while lemma has several uses in other genre like mathematics, philosophy, biology and medical terminology.  

The main uses for headwords

• A headword introduces an entry in a dictionary. It is the way words are sorted in a  dictionary and reference books.
• The headword is the main word in a phrase that the meaning of the phrase centres on.  It can usually be replaced by a pronoun.  

Different uses for lemma

A lemma is not just a word categorisation, but also categorises words within different studies and philosophies.  

• A lemma is also known as a base form of a word and when it is found in a dictionary it is known as the headword.  In lexicography the lemma is the headword used for indexing.  The verb form of a lemma is usually represented in the infinitive of present tense or first person singular.  Different words can be formed from a lemma leading to the creation of basic stem words or lemma words changing with the addition of syllables like ‘tion’ that change the word and add to the lexiology
• A lemma in mathematics is a stepping stone, or minor proven proposition in informal logic and leads to a larger result.  It is known as a ‘helping theorem’ or an ‘axillary theorem.’  
• We can see lemma appearing in biology as a phytomorphological term referring to part of a plant.  
• In philosophy a lemma is a proposition put forward as part of an argument in discussions of knowledge.  In the concept of ‘no false lemmas’  it means a belief will not count as knowledge if the reasoning leading to the belief contains false lemma.   A philosophical statement that is accepted as true is a lemma.  It allows another statement to be proved to be true through the belief in the lemma..
• In biology the lemma is the outer bract encircling flowers in a grass spike-let.

Chart to compare headword and Lemma 

Summary 

Understanding the concept of a headword is fairly simple as it is used to organise words in a dictionary and a reference book.  Understanding the concept of lemma is more complex as lemma comes into philosophy and the significance of the knowledge of words and their meanings.  Lemma back up different theory and mathematical concepts.  Lemma are used to verify the meaning of words.  In terms of their use in the dictionary, lemma and headword are interchangeable.  Lemma are part of all the word forms with the same meaning and in lexicography it is a lemma that is usually the headword used in the dictionary meaning.  The lemma is derived from the root word and if you are considering verbs, the lemma is the present tense of the verb studied.  Lemma are more interchangeable in concept with headwords while headwords stick with their role of an index resource word and the main theme word in a sentence with other adjectives and adverbs to extend the meaning of the main headword.

The linguistic knowledge and analysing of these two words, along with their uses in sentence and in determining the word place in a directionality, make them similar but not the same.  They do have one thing in common with their indexing use in a reference book or dictionary, but other attributes in the part they play in defining other words in their particular contextual purposes are different.

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