1. Meaning
1.1. Historical linguistic is the study of linguistic change
2. Studied areas
2.1. Studying the reconstruction and origin of words.
2.2. Analysis and description of multiple speech communities.
2.3. Tracing (as far as possible) the history of language.
2.4. Describing and analysing changes of any type which have occurred cross-linguistically and within a language itself.
2.5. The construction of a framework of theories which can account for how and why languages change.
3. Concerned of study
3.1. Dialectology
3.1.1. concerns the historical study of dialects.
3.1.1.1. Features which are usually a matter for debate are grammatical variations between two dialects, and phonological changes within a dialect over time.
3.2. Phonology
3.2.1. concerns the study of the sound systems which exist (or have existed) in a specific language.
3.2.1.1. Studies in phonology can also concern comparative approaches to language studies between different time periods, dialects and languages.
3.3. Morphology/Syntax
3.3.1. concerns how the means of expression of a language evolve over time.
3.3.1.1. It is usually focused on inflectional systems, grammatical structures, and word order.
4. Sub-fields
4.1. Comparative Philology (or Comparative Linguistics)
4.1.1. concerns the comparison of cross-linguistic features in order to establish the relatedness of languages.
4.2. Etymology
4.2.1. concerns the study of word histories.