Diving into the Sources in Our World in Data, there's this in a linked PDF:

>Definition: This indicator presents the share of employment which is classified as informal employment in the total economy, and separately in agriculture and in non-agriculture.

>Concepts: Employment comprises all persons of working age who, during a short reference period (one week), were engaged in any activity to produce goods or provide services for pay or profit. Informal employment comprises persons who in their main or secondary jobs were in one of the following categories:

>- Own-account workers, employers and members of producers’ cooperatives employed in their own informal sector enterprises (the characteristics of the enterprise determine the informal nature of their jobs);

• Own-account workers engaged in the production of goods exclusively for own final use by their household (e.g. subsistence farming);
• Contributing family workers, regardless of whether they work in formal or informal sector enterprises (they usually do not have explicit, written contracts of employment, and are not subject to labour legislation, social security regulations, collective agreements, etc., which determines the informal nature of their jobs);
• Employees holding informal jobs, whether employed by formal sector enterprises, informal sector enterprises, or as paid domestic workers by households (employees are considered to have informal jobs if their employment relationship is, in law or in practice, not subject to national labour legislation, income taxation, social protection or entitlement to certain employment benefits). For the purpose of classifying persons into formal or informal employment for this indicator, only the characteristics of the main job are considered. An enterprise belongs to the informal sector if it fulfils the three following conditions:
• It is an unincorporated enterprise (it is not constituted as a legal entity separate from its owners, and it is owned and controlled by one or more members of one or more households, and it is not a quasi- corporation: it does not have a complete set of accounts, including balance sheets);
• It is a market enterprise (it sells at least some of the goods or services it produces);
• The enterprise is not registered or the employees of the enterprise are not registered or the number of persons engaged on a continuous basis is below a threshold determined by the country.