This is a post about words--a few particular (but simultaneously not particular), prevalent words in Vergil (specifically Ecl. 6 & 10) and Latin poetry: nemus, saltus, lucus, and silva. The more I look into them, the more elusive they become. They are difficult to define, but more in the attempt to distinguish them from each other rather than to figure out their essential meanings (saltus aside), partly because they name natural features that are physically close to each other and are interconnected, and partly because different authors seem to use them interchangeably at times, but not at others. All of these words name features of a mountainous, forested landscape (read "Italy" or "Greece") -- in whole or in part, or in some combination thereof. I am in no way offering a solution to this difficulty, but only a few considerations of words that are important to poetry, essential to pastoral, and central for Vergil.
What can be regarded as certain concerning these words? Etymology seems an appropriate place to start:
saltus: origin still unknown. It might be related to a Sanskrit root meaning 'to go'; in the past, it has been proposed that it is related to Latin salire, 'to leap', though I am unsure how prevalent this belief is today. The Sanskrit connection seems plausible in the sense of "mountain pass" (see below).
silva: we are familiar with this one. Like Greek ὕλη, 'wood, woods, forest' etc.
lucus: from the 'light' root, as in lux; so the best English equivalent would seem to be a clearing or a glade (in a wood, of course), where the light shines through the treetops onto the forest-floor; Lewis & Short say "the [S]hining" (capitalization mine)...
nemus: one of the more difficult ones to pin down, though the root is clear: related to Greek words like νέμος, 'wooded pasture, pasture...' and νομός, 'pasture'. The verb νέμω means essentially to divide or distribute. It also gives the words related to νόμος, 'usage, custom, law' etc. For our purposes, if you follow the trajectory of νέμος in Liddell & Scott, the link between 'distribute' and 'pasture, graze' etc. isn't far-fetched. For what is distributed to you becomes, in certain situations, your dwelling-place; you eat in your dwelling-place; if you are a shepherd, what you get are places to feed your herds, thus νέμος = pasture, and the etymological equivalent of nemus.
The ancients were also interested in these words. In fact, Varro had already made the connection between νέμος and nemus in the 1st cent. BC (De Lingua Latina 5.36)...but he also equated nemora with saltus! A brief 2012 article by Vincent Rosivach on Catullus 63 (https://www.jstor.org/stable/41547143?seq=1) discusses three ancient explanations of these words as found in Varro, Festus, and our very own Servius (commentary on Aen., 1.310). For brevity's sake, I will not discuss all of that here, but I will propose a few loose definitions of these words, based on these grammarians, Rosivach's comments, and my own readings. First, here are the brief comments of Varro, Servius, and Festus, with paraphrases/translations (note that Festus and Servius lived centuries after Varro and Vergil, and Servius lived some two centuries after Festus):
Varro: ...quos agros non colebant propter silvas aut id genus, ubi pecus possit pasci, et possidebant, ab usu s<al>vo saltus nominarunt. Haec etiam Graeci νέμη, nostri nemora.
"The fields that they did not cultivate because of silvas or that kind of land where a flock can be put to pasture, they still took possession of and named them saltus from their 'safe/saved use.' (Varro's etymology for saltus.) These the Greeks named νέμη, our nemora."
Servius: ...interest autem inter nemus et silvam et lucum; lucus enim est arborum multitudo cum religione, nemus vero composita multitudo arborum, silva diffusa et inculta.
paraphrase: There is a difference between a 'nemus' and a 'silva' and a 'lucus'; a 'lucus' is a great number of trees with a religious significance, a 'nemus' is a collected (whatever he means exactly by 'composita') multitude of trees, a 'silva' is spread out and uncultivated.
Festus: nemora significant silvas amoenas... (this entry is highly fragmented, and this seems to be the only certain part of the entry; see Rosivach for more information).
"Nemora mean/signify pleasant silvas..."
Some thoughts:
A silva is a larger, more generic word. The requirement for a silva is, naturally, a forest. It seems to be perceived as wilder than a nemus or lucus, but can (always?) contain nemora, luci, and/or saltus. In the Eclogues the silva is what the pastoral poet sings to, and is often associated with shadows and the haunts of beasts.
lucus and nemus, in my experience, seem to be the two that are most synonymous, though perhaps only in the sense of (sacred) grove. Often they are translated as 'groves' in relation to the sacred sort, but Servius implies that a lucus is really the sacred kind. But it is clear that nemus is also used for sacred groves. Strictly, a 'grove' in English is a small(er) collection of trees, not a whole mountain-side of open and forested areas, though the poets seem to mean either of these or other meanings--depending on the occasion--when they use nemora.
nemus it seems is, strictly speaking, a pasture (in a wooded, hilly area), but can also refer to a grove, a sacred grove, or whole area of groves and open spaces. A nemus need not be natural. One could also be planted (cf. the nemus Caesarum).
saltus...Do I even dare? It seems on the one hand to refer vaguely to any given area in a mountainous, wooded land. On the other hand, it can also refer to many specific features within that same land: a small valley, a ravine, defile, mountain pass, woodlands, pastures...which brings me to Vergil.
Eclogue 6 & 10
The topic is too vast for this. I will briefly conclude with a few thoughts on two specific words (nemus and saltus) in two specific places in Vergil's Eclogues:
6.55-6: ...claudite, Nymphae, / Dictaeae Nymphae, nemorum iam claudite saltus...
10.9: Quae nemora aut qui vos saltus habuere, puellae...
These words are of course used elsewhere, but I've chosen these lines because he uses them both in the same sentence on each occasion. The first conclusion it seems one could make is that Vergil, at least, differentiates the two, in that he uses them as separate names in the same thought. 6.55-6: "close, Nymphs, Nymphs of Dicte, close already the saltus of the nemorum..." Here at least, the saltus must be smaller features of the nemorum since they are 'of the nemora'. Because of this, I am tempted to translate it "the passes of the wooded pastures," i.e., almost like "entrances to the nemora, whatever in the world the nemora are here. The narrow passes of the woodland pastures? Passes that lead into the groves? Remember that this line is in reference to keeping out the "roaming footsteps of the bull."
In 10.9, Vergil asks the Naiads what nemora or what saltus kept them from coming to Gallus while he was love-sick. So again, there seems to be a distinction in Vergil's mind between the two. But here the words are equally elusive, if not more. They are distinct, but that is all I can tell. What is clear is that they are both features of the rugged terrain of Greece, the mountain-homes of the Muses. Perhaps the words have more sacred connotations here, considering the reference. But it is pastoral poetry, and in a sense, all nature is sacred, no matter what it is.