A Math function that draws water droplet shape? I just need a quick reference. What is the function for this kind of shape?



Thanks.
Luckily for you, though the admonition of "there's no real reason to expect that an arbitrary shape admits a nice description" is true in general, there is such a curve that might suit your needs: the piriform of Longchamps (piriform - "pear-formed"), with the parametric equation (changed a bit from the equations in the links; I'll leave the proof of equivalence to you):

$$\begin{align*}x&amp;=a(1-\sin\,t)\cos\,t\\y&amp;=b(\sin\,t-1)\end{align*}$$

Here for instance is the case $a=1,\quad b=\frac52$:



On the other hand, remember that in a genuine physical drop, surface tension compels the lower bulbous portion of the drop to be (more or less) spherical; this is of course not the case with the piriform.
This can be considered as a trapezium of very small base and a arc of circle below it.