Trans-Dniester is a narrow region on Ukraine's western border between Moldova and
Ukraine. It broke away from Moldova circa 1991-1992 and has had Russia peacekeeping troops stationed there since. The secession has not been recognized by the UN and Trans-Dniester is shown as part of Moldova on maps.
The BBC reports:
"Pro-Russian politicians and activists in Moldova's breakaway Trans-Dniester region have asked the Russian parliament to draft a law that would allow their territory to join Russia.
The Trans-Dniestrian appeal comes as Moscow moves towards absorbing Crimea into the Russian Federation. Ukraine, the EU and US say that move is illegal.
Russian loyalists dominate Trans-Dniester, with support from Moscow.
The region split from Moldova in a war in 1991-92, as the USSR was collapsing.
Moldova's President Nicolae Timofti said in a news briefing on Tuesday that any decision by Moscow to accept Trans-Dniester "would be a step in the wrong direction".
Wikipedia has an article on the region.
One item of interest is that Trans-Dniester is about 100 miles across the Black Sea from Crimea but is blocked from access to the sea by a narrow strip of Ukrainian land -- about 12 miles. Plus access to the Black Sea from that point is via a large bay about 15 miles long --also part of Ukraine. Given Russia's massive naval advantage, it would be tempting to secure part of Ukraine's western flank with a short amphibious stroll. Plus it might make Moldova think twice about that EU membership.
The other question is how Russia can sustain its current forces in landlocked Trans-Dniester if it does not allow Trans-Dniester to join the Russian Federation, given a now hostile Ukraine.