401k to Roth IRA

It is a rollover because you are moving it from one retirement account to another.  This is important because rollovers do not count or reduce your annual contribution limit.  It is also a conversion because you are changing it from a 401k which is pretax funds to a Roth so you will have to pay the tax on the contribtuions.

Under the tax code it is referred to as a "taxable rollover."  In the tax code, the term "conversion" refers only to the movement of funds from a traditional IRA to a Roth IRA.

Still, due to the similarity in moving funds from a traditional retirement account to a Roth IRA, taxable rollovers are often referred to colloquially as conversions.  However, you'll see it reported on Form 1040 lines 5a and 5b with the ROLLOVER notation, with the taxable amount from box 2a of the Form 1099-R being included on line 5b.