Three parts, none of them exclusive to it
An éclair is choux pastry piped long, filled with crème pâtissière, and finished with a poured glaze, classically chocolate or coffee. Every one of those three is specified elsewhere on this site, and that is the most useful thing to say about it.
The shell is the same paste as a chouquette: butter, water, flour and eggs. Piped round and left empty with pearl sugar on it, that paste is a chouquette; piped long and filled, it is this. The filling is the base of the whole cream family, being milk + egg + sugar + starch, cooked. The glaze is a poured fondant, which has its own single critical temperature.
So an éclair is an assembly, and the practical consequence is diagnostic: a failure belongs to one of the three parts, and knowing which one narrows what to change.
Reading a failure back to its part
A collapsed shell is a choux problem: the paste, the piping or the oven, and nothing to do with what went inside it. A filling that weeps in the fridge overnight is a cream problem, and the cream page names the usual cause, which is whisking a finished pastry cream hard after cooking tears the starch network that was holding it. A glaze that sets dull or grainy is a fondant problem, and fondant is governed by one temperature.
Which is why the assembly framing is worth more than a description. “My éclairs went wrong” has three quite separate answers depending on which layer went wrong, and they do not overlap.
One cross-part rule does apply, and it comes from the cream dataset: adding an acidic ingredient to a starch-set cream before it has set degrades it, so acid belongs at the end. A fruit-flavoured éclair filling is where that bites.
The glaze has a temperature, and it is the one number here
Of the three parts, the glaze is the one with a stated figure attached, and the dataset that records it names éclairs as its use. A fondant pâtissier, the white poured glaze, is worked warm to about 37 °C and never above it: past that it loses its shine. Some references work it nearer 32 °C with a little sugar syrup.
Two instructions come with that, and both are about not breaking it. Loosen it with syrup, not water, if it is too stiff, because water thins it by diluting the sugar rather than by making it flow. And stir it with a spatula, never a whisk, because beating air into it is what turns a glaze dull.
So a dull or streaky top has a short list of causes, and all of them are temperature or handling rather than recipe. The full specification, including the three separate things the word fondant names, is at white fondant icing.
Why the shape has a name
Éclair means lightning, and explanations for the name vary between the glaze’s shine, or the speed with which one is eaten. Nothing consulted for this page settles which, so neither is asserted here. The shape itself is straightforwardly practical: a long shell holds more filling per bite than a round one, and a flat glazed top is a surface a round bun does not have.
The rest of the choux family, the six creams and the blind-bake specifications are collected in the technique section.
Questions about éclairs
What is an éclair made of?
Three things, all specified separately on this site: choux pastry of butter, water, flour and eggs, piped long, a crème pâtissière filling, and a poured glaze. Nothing in it is unique to an éclair.
Why did my éclair filling go runny overnight?
That is a pastry-cream problem rather than an éclair one. The usual cause is whisking the finished cream hard after cooking, which tears the starch network holding it together.
What is the difference between an éclair and a profiterole?
The piping and the finish. The same choux paste piped long, filled with pastry cream and glazed is an éclair; piped round and filled with cream or ice cream is a profiterole.
Why is it called an éclair?
Éclair means lightning, and the explanations vary between the shine of the glaze and how fast one is eaten. Nothing consulted settles which, so neither is asserted here.