Or frangipani as it’s also called. Good question. To put it succinctly, it’s a filling, one that’s frequently flavored with nuts (usually almonds, but also hazelnuts or pistachios) and used in pies, tarts and cakes. It’s egg-based, which puts it in the custard family, though of the “still” (i.e. baked) variety.
That’s as specific a definition as I can come up with, because truth be told, “frangipane” is one of the most varied substances in all of pastrydom. On one end of the texture spectrum, it can be an nut-flavored pastry cream, composed of whipped eggs, sugar, butter and almond or hazelnut paste. But franginpane can get much, much firmer depending on what you add to it. Put in a little flour and swap out the nut paste for actual ground almonds, pistachios or hazelnuts and you get a very batter-like device, one that bakes up more cakey than creamy. Add still more flour and nuts and the result can be a firm tart crust.
In pear tarts, frangipane varies from the very light to the somewhat cakey. It often contains ground almonds and a small amount of flour, though purists prefer no flour at all. I like a little flour, personally, both for the texture and the added insurance it provides against “breaking”.