Language attrition is the process of decreasing proficiency in or losing a language. For first or native language attrition, this process is generally caused by both isolation from speakers of the first language ("L1") and the acquisition and use of a second language ("L2"), which interferes with the correct production and comprehension of the first. Such interference from a second language is likely experienced to some extent by all bilinguals, but is most evident among speakers for whom a language other than their first has started to play an important, if not dominant, role in everyday life; these speakers are more likely to experience language attrition. It is common among immigrants that travel to countries where languages foreign to them are used. Second language attrition can occur from poor learning, practice, and retention of the language after time has passed from learning. This often occurs with bilingual speakers who do not frequently engage with their L2.

Several factors affect language attrition. Frequent exposure and use of a particular language is often assumed adequate to maintain the native language system intact. However, research has often failed to confirm this prediction. A person's age can predict the likelihood of attrition; children are demonstrably more likely to lose their first language than adults. The process of learning a language and the methods used to teach it can also affect attrition. A positive attitude towards the potentially attriting language or its speech community and motivation to retain the language are other factors which may reduce attrition. These factors are too difficult to confirm by research.

These factors are similar to those that affect second-language acquisition and the two processes are sometimes compared. However, the overall impact of these factors is far less than that for second language acquisition.

Language attrition results in a decrease of language proficiency. The current consensus is that it manifests itself first and most noticeably in speakers' vocabulary (in their lexical access and their mental lexicon), while grammatical and especially phonological representations appear more stable among speakers who emigrated after puberty.

The study of language attrition became a subfield of linguistics with a 1980 conference at the University of Pennsylvania called "Loss of Language Skills". The aim of the conference was to discuss areas of second language attrition and to discuss ideas for possible future research. The conference revealed that attrition is a wide topic, with numerous factors and taking many forms.

Decades later, the field of first language attrition gained new momentum with two conferences held in Amsterdam in 2002 and 2005, as well as a series of graduate workshops and panels at international conferences, such as the International Symposium on Bilingualism (2007, 2009), the annual conferences of the European Second Language Association, and the AILA World Congress (2008). The outcomes of some of these meetings were later published in edited volumes. The term first language attrition (FLA) refers to the gradual decline in native language proficiency. As speakers use their L2 frequently and become proficient (or even dominant) in it, some aspects of the L1 can deteriorate or become subject to L2 influence.

Research on L2 attrition is lacking, as most research focused on L1 attrition. Only during the 1970s and early 1980s did research on L2 attrition and memory start to appear. However, there are many overlaps between L1 attrition and L2 attrition.

To study the process of language attrition, researchers initially looked at neighboring areas of linguistics to identify which parts of the L1 system attrite first; lacking years of direct experimental data, linguists studied language contact, creolization, L2 acquisition, and aphasia, and applied their findings to language acquisition. Language loss caused by aging, brain injuries, or neurological disorders is not considered part of language attrition.