Non-linguists usually use "phonetic" to refer to certain kinds of spelling systems, whereas linguists have a more technical concept in mind which they use the word "phonetic". (Usually we use the term to refer to how a language is pronounced, but some languages like ASL aren't pronounced. But the expression "physically realized" is wrong, because we don't talk about written text as being the "phonetic output"). The popular non-linguist concept refers to (1) the extent to which the pronunciation can be predicted from the spelling and (2) the extent to which the spelling can be predicted from the pronunciation. English is "not phonetic" on both counts. Finnish is highly phonetic on both counts. Predicting spelling in French based on pronunciation is very hard (like English), but reading a French text is much less challenging. But even or perhaps especially in a "phonetic" language like Finnish, you have to learn spelling-to-orthography rules.
Since actual pronunciation is highly variable and people don't change their spellings depending on which of a dozen pronunciation of "traitor" they use at a particular time (unless they are authors trying to convey a regional accent), the idea that there is a single pronunciation of a word in a language is mistaken. But non-linguists who talk about "phonetic languages" aren't concerned with the range of linguistic variation that exists in a society or within an individual, instead there is some presumed standard, and that is what phoneticity is judged on the basis of.
As for Sanskrit, it is standardly written in Devanagari so we don't have to say that Sanskrit written in Devanagari is "phonetic". But is you want to include Sanskrit as written in other scripts, that could be necessary. There is a transliteration scheme for Devanagari where पाणिनि converts to Pāṇini, but Latin based spelling would be "Panini". I do not know if there is a perfect bidirectional transliteration for Sanskrit into Tamil or Bangla (etc.) alphabets. Incidentally, Sanskrit is a good example of how "phonetic" writing (in the popular sense) is not necessarily phonemic, because the spelling system includes sub-phonemic detail (visarga, the bindu and chandrabindu used for nasalization).