Tip In
1.Noun: The point at which the tip of the penis enters the vagina.

2.Verb: The act of putting the tip of the penis into the vagina.

3. The promise you make during 3rd base. 
i.e. " I'll only put the tip in." 
 
" Stacey, baby,I don't think we are ready to have sex either, I just want to tip in, and then I'll pull away, that's all. Promise"