If you were to interpret the word "pluvial" literally, you might conclude that a pluvial lake is simply one filled primarily from rainfall. But you'd be missing the point.
The vast changes in Earth's climate which we now call "Ice Ages" manifested themselves in more ways than just continent-covering ice sheets: Places that are arid today experienced considerably more rainfall then than now. In places of internal drainage such as the Basin and Range country of western North America, rainwaters collected to form lakes of all sizes. As the Earth warmed up into the Holocene, rainfall decreased and the lakes dried up. Geomorphologists can tell the lake existed by erosional and sedimentary features they leave behind, such as fossil beaches (often hundreds of feet above a current lake level), varved sedimentary layers from the former lake bed, and deltas from streams that once emptied in to the lake but are now high and dry.
"Pluvial lake" refers to a lake that existed in the past, or was much larger than it is currently, during a period of increased rainfall, whether during an Ice Age or a smaller, more recent climate fluctuation. Lake Bonneville, which covered parts of Utah, Nevada, and Idaho is a pluvial lake; it never saw a glacier and could only have formed via rainfall.
However, not every ancient lake was a pluvial lake. Proglacial lakes such as Lake Missoula and Lake Agassiz were formed by ice dams, and were filled primarily by glacial meltwater. Other lakes can be labeled "pluvial", but it is more accurate to say the areas went through periods of pluvial conditions. A good example is Lake Texcoco (which has been mostly filled in with Mexico City). The Little Ice Age was in full swing just as the Mexica were settling Tenochtitlan, and increased rainfall probably swelled this playa lake to the level seen in artists' renditions of the Aztec capital, but Texcoco is (was?) a remnant of a much larger and deeper lake that existed during the last Ice Age.