When it rains, puddles are formed on the streets. Is there a classical Latin word for such a small puddle? The only two examples of "puddle" in L&S are fetutina (a dirty and stinking puddle) and lustrum (more like a swamp), and both seem to be off the mark. After more searching, I found stagnum and lacuna, but neither seems to hit the nail on the head. Perhaps stagnum sounds most promising, but what word was actually used in classical Latin?
Of the small puddles that collect after rain, classical Latin seems to prefer a small collection of water. For example, Lucretius uses conlectus aquae of a shallow puddle between paving stones, much as one would find after a recent downpour:
At conlectus aquae digitum non altior unum,
qui lapides inter sistit per strata viarum,
despectum praebet sub terras impete tanto,
a terris quantum caeli patet altus hiatus,
nubila despicere et caelum ut videare et aperta
corpora mirande sub terras abdita cernas.
But a puddle of water no deeper than one finger, lying between the stones upon a paved street, offers a view downwards under the earth to as great a reach as the open heavens yawn on high, so that you seem to look down upon the clouds and heaven, and you see manifest objects miraculously buried beneath the earth.
De Rerum Natura, 4.414-419 (trans. W. H. D. Rouse)
Similarly, Apuleius uses conceptaculum aquae, specifically in the context of recent rain:
Nam forte pluviae pridianae recens conceptaculum aquae lutulentae proximum conspicatus ...
It happened to have rained the day before, and I spotted a new puddle of muddy water nearby ...
ADDIT Stagnum, on the other hand, seems to refer to something more like a pool of water or, at least, to a body of water larger than a puddle.
For example, Columella, writes of the importance of having a supply of water for pigs:
Sed cum omni quadrupedi per aestatem sitis sit infesta, tum suillo maxime est inimica. Quare non ut capellam vel ovem, sic et hoc animal bis ad aquam duci praecipimus: sed si fieri potest, iuxta flumen aut stagnum per ortum Caniculae detineri
While thirst in the summer is pernicious to all quadrupeds, it is specially hurtful to pigs. We, therefore, advise that they should not be taken to water twice a day, like goats and sheep, but that, if possible, they should be kept in the neighbourhood of a river or pool at the time of the rising of the Dogstar
On Agriculture, 10.7.6
And here, speaking of fish ponds:
Stagnum censemus eximie optimum, quod sic positum est, ut insequens maris unda priorem submoveat, nec intra conseptum sinat remanere veterem
We consider that incomparably the best pond [for fish] is one which is so situated that the incoming tide of the sea expels the water of the previous tide and does not allow any stale water to remain within the enclosure
8.17.1 See also 8.14.2 re: providing water for geese.
Here, in Caesar's The African War, he clearly refers to something other than a puddle:
Erat stagnum salinarum, inter quod et mare angustiae quaedam non amplius mille et D passus intererant
There was a lagoon of salt water, separated from the sea by a certain narrow strip of land not more than a mile and a half wide
Caesar, The African War, 80
Ovid, at Metamorphoses, 4.297 (and ff), uses stagnum of the pool in which Salmacis bathes her limbs. And again, at 5.411 (but read 409 ff for story), he speaks of Cyane, standing up to her waist in a stagnum.
Lastly, Virgil specifies that they are deep:
Cocyti stagna alta vides Stygiamque paludem
[Anchises], you are looking at the deep pools of Cocytus and the Stygian marsh,
Lacuna usually implies some kind of gap — if you like, a gap into which water runs and then stagnates, when it would become a puddle. The verb stagno, -are means 'form into pools', and the associated stagnum is any sort of standing water, of any shape or area.
I can't see any problem in using stagnum. It would surely cause no eyebrows to be raised if you were to modify it to a diminutive, say stagnulum or stagnella, either of which should be immediately understood.