See, we don't wear pants. The Bible says women shouldn't wear men's clothing and that men shouldn't wear women's clothing. Pants are for cootie-ridden boys. So, in order to justify our (and I say "our" because I raise my hand as one of the participants) donning a pants-like garment in good conscience, some explanation is in order.
Pants by day, pajama bottoms by night
If a woman were to wear a two-legged garment outside in the daylight, those are pants, hands down. No contest there. At that point, she may as well climb into the handbasket on its way to you-know-where. But if she wears a two-legged garment inside her house, they morph into a shaky but, depending on the amount of fire in the preaching she's used to, acceptable form of non-public attire. However, at night, she can breathe a sigh of relief and rest assured. They're safe, feminine pajama bottoms. Worn to bed and nowhere else.
And really, they have to be some form of pajamaesque two-legged garment that you would actually wear to bed. A pajama ensemble is preferred. Sweatpants, perhaps. But jeans? Girl, SAT down and go pray through.
Defining something like this is definitely not a science, but I think it's safe to say that the public/private dichotomy along with the amount of light outside are good indicators of acceptability.
The 5-year-old dilemma
But, this justification comes not without a few snags. For example, try explaining this to a 5-year-old.
5-year-old: (Upon seeing me in pajama bottoms) Ooh, you're wearing pants!
Me: These aren't pants, silly. They're pajama bottoms.
5-year-old: But they're pants.
Me: No, no, no. Listen. Pants are what boys wear. I'm a girl, and I'm wearing pajama bottoms.
5-year-old: But they look like pants.
Me: (sighing frustratedly) Pants are what boys wear outside. But girls can wear pajama bottoms to bed.
5-year-old: They're pants.
You see? 5-year-olds just don't get nuance.