Lieux de mémoire or sites of memory are places or objects which act as repositories of memories. The term was coined by French historian Pierre Nora.

Pierre Nora defined "lieux de mémoire" as material or abstract entities, like monuments or symbols, that anchor collective memory and shape a community’s shared identity.
French historian Pierre Nora (1931-2025) coined the term "lieux de mémoire" (sites of memory) in the late 1970s to describe material or abstract objects that carry memories of the past into the present. Between 1984 and 1992 Nora edited three volumes of his famous work titled Les Lieux de Mémoires which listed important sites linked to French national identity. Nora's sites of memory is an essential concept for everyone interested in history and the broader fields of reception and memory studies.

Clarification: You will find the term used both as lieu de mémoire (plural: lieux de mémoires) and as sites of memory.
Here is a detailed definition of the term in Nora's own words:
"A "lieu de mémoire" is any significant entity, whether material or non-material in nature, which by dint of human will or the work of time has become a symbolic element of the memorial heritage of any community (in this case, the French community)" (Nora 1996, XVII)
This brief but rather concise defintion is also useful:
"liex de mémoire where memory crystallizes and secretes" (Nora 1989, 1).
It is important to clarify here that with the term memory, Nora refers to collective memory (i.e, common memories that shape the identity of groups of people).

A simple example is that of the National September 11 Memorial, a space that acts as a repository for the memory of the 9/11 attacks of 2001. The memorial is not only keeping the memory alive but also shapes the way the memory itself is preserved (in this case the memory is crystallized as a physical void in the exact spece where the twin towers once stood) so that it can be effectively accessed by a wider audience.
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For Nora, a lieu de mémoire could be anything from a monument to a color.
Here are some key examples:

Nora believed that there is a divide between (collective) memory, a spontaneous memory of the past kept alive by the public, and history, an institutionalized form of memory that the elites enforce. For Nora, as history progresses, memory grows weaker, history grows stronger and seeks to monopolize the past. For Nora, the sites of memory are places that belong to history, but contain a fraction of the memory that was lost (sort of the result of a typical dialectical scheme where a thesis (collective memory) loses the battle against an antithesis (history) and the result of the struggle is a synthesis (lieu de mémoire) that contains parts of what was lost and what won.
However, here it should be noted, that historians after Nora, tend to view history as a form of memory that constructs the past as much as it seeks to institutionalize it. So Nora's dualistic appraoch that positioned history against memory is considered rather obsolete.
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These examples demonstrate how broad Nora's definition of a site of memory could be. In fact, some critics have argued that the term is so loose that it means nothing (see Olick and Robbins 1998). Truly the term can mean anything, from a word to a language, from a symbol to a tradition, from a historical event to a landscape and from a parade to even a whole province can be considered sites of memory if a researcher deems them to hold the symbolic capital.
What a critic perceives as a weakness, someone else can see as a strength as it allows a researcher to isolate spaces and study them as standalone units of collective memory.