In
phonetics, the process of
softening a
consonant. This is actually as vague as it sounds. There is no definite meaning of "soft" in
linguistics: a sound is conceived as soft (or
lenis) if it takes less effort to
articulate than another.
In Latin the word for 'father' was patre (in Classical Latin this was an oblique case form). The T is voiceless, but the A and R around it are voiced, so to make the T you have to interrupt a continuous voicing. It is easier to keep the voicing throughout, making a voiced D: padre. This of course is common in the Romance languages deriving from Latin. The change T > D is an example of lenition.
Now D is still an interruption. A further change that took place in Spanish and French was to make the D continuous, a fricative rather than a stop. The Spanish sound of D here or between vowels is a weak sound resembling English TH in then. This also is lenition. (So you see it has no one definite meaning.) In modern Spanish the D may be so weak as almost to have disappeared; and in Old French it did: so père.
In Welsh lenition is another name for soft mutation, which changes initial voiceless consonants to voiced, and voiced to fricative under certain grammatical circumstances. So telyn 'harp' and dinas city are lenited in ei delyn 'his harp' and ei ddinas 'his city': DD is pronounced like English TH in then. So the changes have occurred here as historically they did in the evolution of Latin.
In Ancient Hebrew lenition took place in the so-called beghadhkephath consonants, a mnemonic for b g d k p t, which became bh gh dh kh ph th in some positions. (Here bh ph mean v f.) By "Ancient" Hebrew I am hedging my bets. It is not clear that the phonetic changes had yet taken place in the living language when Hebrew-speakers wrote down the Bible, but it had occurred by the time later grammarians devised the Masoretic vowelling and punctuation. This change took place between vowels except when double, and at the end of a syllable.
In Finnish similar changes happen depending on whether a syllable is open or closed. A double plosive becomes single when the syllable is closed with a consonant: hoikka pappi 'a slender priest', plural hoikat papit. The corresponding single consonants also soften in the same circumstances: parka kelta tupa 'a poor yellow cottage', parat kellat tuvat 'poor yellow cottages'.
All these changes are examples of shifting down a scale of hardness. I can't think of a valid phonetic definition of hard or soft, but it's undeniable that a scale like this appears in many languages or their histories. Hardest is the double voiceless stop such as [pp], then come single voiceless [p], voiced stop [b], voiced fricative [β] or [v], then a frictionless version (as in modern Spanish), and finally complete disappearance. Likewise we get the sequence [tt] > [t] > [d] > [ð] > [ ]; and likewise with [kk] down through [g] to nothing.